Move ANGLEWebKitBridge to platform/graphics/angle/ANGLEPlatformBridge.

Part of the effort to migrate platform out of core.

R=abarth@chromium.org, abarth, eae, senorblanco
BUG=303332

Review URL: https://codereview.chromium.org/25825003

git-svn-id: svn://svn.chromium.org/blink/trunk@158988 bbb929c8-8fbe-4397-9dbb-9b2b20218538