gpu: Remove WebGraphicsContext3D::makeContextCurrent() at all.

Now chromium initializes and binds the graphics context to the current thread before
returning it to Blink. So get rid of all makeContextCurrent().

BUG=404121

Review URL: https://codereview.chromium.org/518003002

git-svn-id: svn://svn.chromium.org/blink/trunk@181113 bbb929c8-8fbe-4397-9dbb-9b2b20218538