Use DEF_GPUTEST_FOR_*_CONTEXT macros to obtain the
test GPU context.
Makes changing the context -related classes easier,
since not all tests need to be changed.
BUG=skia:2992
Review URL: https://codereview.chromium.org/1448873002
This fix is necessary to correctly propagate invalidations that
are external to skia. For example, when drawing video or WebGL
into a 2D canvas in Chrome, with mipmaps enabled.
BUG=crbug.com/498356
TEST=GrTextureMipMapInvalidationTest
Review URL: https://codereview.chromium.org/1177843007