[build] Move split_static_library.gni from Chromium repo

We'll remove the file from Chromium in a follow up after V8 has rolled
+ 2 days.

Bug: v8:9911
Change-Id: I69fe56855f1ba83bec0d39e0fb6acb7e4182c6b7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1897826Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64742}

gni/split_static_library.gni

+# Copyright 2019 the V8 project authors. All rights reserved.
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/config/compiler/compiler.gni")
+
+template("split_static_library") {
+  assert(defined(invoker.split_count),
+         "Must define split_count for split_static_library")
+
+  # In many conditions the number of inputs will be 1 (because the
+  # count will be conditional on platform or configuration) and for
+  # some build configurations it's unnecessary to split libraries
+  # since the tooling will never create files of a problematic size.
+  if (invoker.split_count == 1 || use_lld) {
+    static_library(target_name) {
+      forward_variables_from(invoker, "*")
+    }
+  } else {
+    group_name = target_name
+
+    generated_static_libraries = []
+    current_library_index = 0
+    foreach(current_sources, split_list(invoker.sources, invoker.split_count)) {
+      current_name = "${target_name}_$current_library_index"
+      assert(
+          current_sources != [],
+          "Your values for splitting a static library generate one that has no sources.")
+      generated_static_libraries += [ ":$current_name" ]
+
+      static_library(current_name) {
+        # Generated static library shard gets everything but sources (which
+        # we're redefining) and visibility (which is set to be the group
+        # below).
+        forward_variables_from(invoker,
+                               "*",
+                               [
+                                 "check_includes",
+                                 "sources",
+                                 "visibility",
+                               ])
+        sources = current_sources
+        visibility = [ ":$group_name" ]
+
+        # When splitting a target's sources up into a series of static
+        # libraries, those targets will naturally include headers from each
+        # other arbitrarily. We could theoretically generate a web of
+        # dependencies and allow_circular_includes_from between all pairs of
+        # targets, but that's very cumbersome. Typical usage in Chrome is that
+        # only official Windows builds use split static libraries due to the
+        # Visual Studio size limits, and this means we'll still get header
+        # checking coverage for the other configurations.
+        check_includes = false
+
+        # Uniquify the output name if one is specified.
+        if (defined(invoker.output_name)) {
+          output_name = "${invoker.output_name}_$current_library_index"
+        }
+      }
+
+      current_library_index = current_library_index + 1
+    }
+
+    group(group_name) {
+      public_deps = generated_static_libraries
+      forward_variables_from(invoker,
+                             [
+                               "testonly",
+                               "visibility",
+                             ])
+    }
+  }
+}
+
+set_defaults("split_static_library") {
+  configs = default_compiler_configs
+}

gni/v8.gni

@@ -4,7 +4,7 @@
 
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/v8_target_cpu.gni")
-import("//build/split_static_library.gni")
+import("split_static_library.gni")
 
 declare_args() {
   # Set flags for tracking code coverage. Uses gcov with gcc and sanitizer