Remove allocation and sync point / query structs from blink API

These structs and fields were added to WebGraphicsContext3D purely to
support the compositor.  WebGL and accelerated skia canvas don't use them.
Now that the compositor is refactored to not use these (r230043 / r231527),
they can be removed from the Blink API.

Review URL: https://codereview.chromium.org/51943002

git-svn-id: svn://svn.chromium.org/blink/trunk@160959 bbb929c8-8fbe-4397-9dbb-9b2b20218538