Refactor ToBlinkStringView() to be faster.

The last implementation accidentally caused an AtomicString to be
created even if externalization was not possible. The new code only
generates an AtomicString when externalization can happen.

This is broken out of
  https://chromium-review.googlesource.com/c/chromium/src/+/2204539

which in turn is broken out of
  https://chromium-review.googlesource.com/c/chromium/src/+/1557854

Bug: 1083392
Change-Id: I091872fcd0f8cf26d6d6e1a0fac4a3e115f7543b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2268584Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784499}

third_party/blink/renderer/platform/bindings/string_resource.h

@@ -254,6 +254,13 @@ PLATFORM_EXPORT StringView ToBlinkStringView(v8::Local<v8::String>,
 
 PLATFORM_EXPORT String ToBlinkString(int value);
 
+// The returned StringView is guaranteed to be valid as long as `backing_store`
+// and `v8_string` are alive.
+PLATFORM_EXPORT StringView
+ToBlinkStringView(v8::Local<v8::String> v8_string,
+                  StringView::StackBackingStore& backing_store,
+                  ExternalMode external);
+
 }  // namespace blink
 
 #endif  // THIRD_PARTY_BLINK_RENDERER_PLATFORM_BINDINGS_STRING_RESOURCE_H_