[build] Add v8_trace_ignition build option

Since --trace-ignition now has to be enabled at snapshot-building time,
this patch adds it as a gn build option.

Change-Id: I5d55339a7be7eef4e1f9da46ec44fbfd431325b7
Reviewed-on: https://chromium-review.googlesource.com/474905Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44583}

BUILD.gn

@@ -72,6 +72,9 @@ declare_args() {
   # Sets -dV8_ENABLE_CHECKS.
   v8_enable_v8_checks = ""
 
+  # Builds the snapshot with --trace-ignition
+  v8_trace_ignition = false
+
   # With post mortem support enabled, metadata is embedded into libv8 that
   # describes various parameters of the VM for use by debuggers. See
   # tools/gen-postmortem-metadata.py for details.
@@ -732,6 +735,10 @@ action("run_mksnapshot") {
     ]
   }
 
+  if (v8_trace_ignition) {
+    args += [ "--trace-ignition" ]
+  }
+
   if (v8_use_external_startup_data) {
     outputs += [ "$root_out_dir/snapshot_blob.bin" ]
     args += [