Additional DCHECKs when accessing SharedArrayBuffer

This change makes wtf::ArrayBuffer::data() require that the underlying
ArrayBuffer be non-shared. The caller must use dataShared() or
dataMaybeShared() to access the underlying data of a SharedArrayBuffer.

BUG=678410

Review-Url: https://codereview.chromium.org/2617073002
Cr-Commit-Position: refs/heads/master@{#443472}
6 files changed