These sites don't necessarily need the full power of a GrRenderTarget object. This is a clean up for switching over to GrRenderTargetProxys.
Review URL: https://codereview.chromium.org/1486923004
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