gpu: Remove set_testing_force_is_angle

It's needed for testing GL_ANGLE_depth_texture as well as
the logic (incl. texture clear logic) to prefer texSubImage2D
over texImage2D for full updates.

This changes things into feature and workaround flags, respectively,
which now also allows to set texsubimage2d_faster_than_teximage2d
for other drivers.

R=jbauman@chromium.org, zmo@chromium.org

Review URL: https://codereview.chromium.org/68113014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235238 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235238