Encapsulate functionality of the mock GL interface to MockGLInterface

Previously, the mock GL interface was split into two classes, GLInterface and MockGLInterface, and related functionality existed also outside these classes. Grouping the functionality under MockGLInterface and removing the ability to read the static GLInterface* from outside makes this part of the code more self-contained and easier to understand.

The comment explaining the role of MockGLInterface is also fixed: the interface is not the desktop GL interface, but rather an union of different GL interfaces.

Also clean up unused autogenerated egl/glx/osmesa/wgl headers. Only the gl interface header is currently being used for mocking.

The commit doesn't introduce functional changes, but only refactors existing functionality.

BUG=none
TEST=gpu_unittests

Review URL: https://codereview.chromium.org/132823009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247086 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247086