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With this CL we finally use v8::BackingStore as the backing store
implementation of blink::ArrayBuffer.

Changes:
* Use v8::BackingStore as the backing store for ArrayBufferContents.
* Remove ArraybufferContents::DataHolder and
  ArrayBufferContents::DataHandle.
* Replace all uses of DataHolder in ArraybufferContents with uses of
  v8::BackingStore.
* Replace the use of DataHandle in the V8 bindings.
* Add an exception to the presubmit check to allow std::shared_ptr in
  ArrayBufferContents. It is contained there and does not escape.

R=haraken@chromium.org, ulan@chromium.org

Bug: chromium:1008840
Change-Id: Iffa31e889daf4ae54de96f55b4612833d096a884
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1886858
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712571}