Kill Source/core/platform

This moves the last bits of core/platform (WebArrayBuffer and WebCryptoResult)
into blink_platform.

Depends on chromium patch https://codereview.chromium.org/149133005/

R=abarth@chromium.org
BUG=297477

Review URL: https://codereview.chromium.org/139133005

git-svn-id: svn://svn.chromium.org/blink/trunk@166660 bbb929c8-8fbe-4397-9dbb-9b2b20218538