memory-infra: Reports per-isolate size of scripts source.

This reports the total size of source code size for all external scripts
in a V8 isolate. As with other V8 code stats, this doesn't distinguish
between live and dead objects and doesn't scan the young generation.

Bug: 837659
Change-Id: I746729f3c167cbad847599b4b128fa517a116c7e
Reviewed-on: https://chromium-review.googlesource.com/1032936Reviewed-by: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559473}

gin/v8_isolate_memory_dump_provider.cc

@@ -79,6 +79,10 @@ void DumpCodeStatistics(
       "bytecode_and_metadata_size",
       base::trace_event::MemoryAllocatorDump::kUnitsBytes,
       code_statistics.bytecode_and_metadata_size());
+  heap_spaces_dump->AddScalar(
+      "external_script_source_size",
+      base::trace_event::MemoryAllocatorDump::kUnitsBytes,
+      code_statistics.external_script_source_size());
 }
 
 // Dump the number of native and detached contexts.