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
BUG=skia: Review URL: https://codereview.chromium.org/1413973002
Review URL: https://codereview.chromium.org/1264003002