Simplify ArrayBuffer::setDeallocationObserver protocol.

Currently all clients need to manually call
v8::V8::AdjustAmountOfExternalAllocatedMemory, so they all depend on V8
API. The patch hides this and also makes the protocol more simple.

BUG=

Review URL: https://chromiumcodereview.appspot.com/24160006

git-svn-id: svn://svn.chromium.org/blink/trunk@158186 bbb929c8-8fbe-4397-9dbb-9b2b20218538