[build] Enable hooking up custom_deps BUILD.gn files.

This adds a new gn arg v8_custom_deps to add a dependency that's dynamically
added under v8/custom_deps. E.g. for depending on a target in
v8/custom_deps/test_root/BUILD.gn set
v8_custom_deps = "custom_deps/test_root:target_name".

Bug: chromium:846711
Change-Id: If03d48dba07cb36dc1b981602e5dc267deec086b
Reviewed-on: https://chromium-review.googlesource.com/1095218Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53640}

BUILD.gn

@@ -22,6 +22,9 @@ declare_args() {
   # Print to stdout on Android.
   v8_android_log_stdout = false
 
+  # Dynamically set an additional dependency from v8/custom_deps.
+  v8_custom_deps = ""
+
   # Turns on deprecation warnings for HeapObject::GetIsolate,
   # HeapObject::GetHeap, Handle(T* obj) and handle(T* obj).
   v8_deprecate_get_isolate = false
@@ -3195,6 +3198,11 @@ group("gn_all") {
     "tools:gn_all",
   ]
 
+  if (v8_custom_deps != "") {
+    # Custom dependency from directory under v8/custom_deps.
+    deps += [ v8_custom_deps ]
+  }
+
   if (want_v8_shell) {
     deps += [ ":v8_shell" ]
   }