bindings: Moves platform/WebArrayBuffer to web/WebArrayBuffer.

      Not all the clients, but some of the clients of WebArrayBuffer expect
WebArrayBuffer to be a DOM instance of type 'ArrayBuffer', which is
core/dom/DOMArrayBuffer in our codebase.

So, let's move public/platform/WebArrayBuffer.h to
public/web/WebArrayBuffer.h and use DOMArrayBuffer as the underlying
implementation.

We'll keep public/platform/WebArrayBuffer.h for a while to keep
compatibility, but the clients should transition to
public/web/WebArrayBuffer.h .

BUG=235436

Review URL: https://codereview.chromium.org/767543002

git-svn-id: svn://svn.chromium.org/blink/trunk@186279 bbb929c8-8fbe-4397-9dbb-9b2b20218538