gpu: Remove WebGraphicsContext3D::makeContextCurrent() in Blink

makeContextCurrent() has two roles;
1. Initialize.
2. Makes the OpenGL context current on the current thread.

#2 is the legacy of WebKit and chromium GPU architecture doesn't require it.
So all #2 usages are removed.

BUG=404121

Review URL: https://codereview.chromium.org/481433002

git-svn-id: svn://svn.chromium.org/blink/trunk@181032 bbb929c8-8fbe-4397-9dbb-9b2b20218538