Caching the result of readPixelsSupported

The call was calling GR_GL_GetIntegerv 2 times for each readPixels
and thus was causing a loss of performance

(resubmit of issue 344793008)

Benchmark url: http://packages.gkny.fr/tst/index.html

BUG=skia:2681

Committed: https://skia.googlesource.com/skia/+/753a2964afe5661ce9b2a8ca77ca9d0aabd3173c

Committed: https://skia.googlesource.com/skia/+/8339371f1ec3c57a0741932fd96bff32c53d4e54

Review URL: https://codereview.chromium.org/364193004
diff --git a/gyp/tests.gypi b/gyp/tests.gypi
index e4a46f2..1d38c48 100644
--- a/gyp/tests.gypi
+++ b/gyp/tests.gypi
@@ -193,6 +193,7 @@
     '../tests/StrokeTest.cpp',
     '../tests/SurfaceTest.cpp',
     '../tests/TArrayTest.cpp',
+    '../tests/THashCache.cpp',
     '../tests/TLSTest.cpp',
     '../tests/TSetTest.cpp',
     '../tests/TextBlobTest.cpp',