Implement a benchmark for GrResourceCache

Adds "grresourcecache_add" and "grresourcecache_find" bench tests to test
GrResourceCache::add and GrResourceCache::find. The tests work only
with GPU backends, since GrResourceCache needs an GrGpu.

Modifies bench tests to override SkBenchmark::isSuitableFor(Backend)
function that specifies what kind of backend the test is inteded
for. This replaces the previous "fIsRendering" flag that would
indicate test that did no rendering.

Adds SkCanvas::getGrContext() call to get the GrContext that the
canvas ends up drawing to. The member function solves a common
use-case that is also used in the benchmark added here.

R=mtklein@google.com, bsalomon@google.com

Author: kkinnunen@nvidia.com

Review URL: https://codereview.chromium.org/73643005

git-svn-id: http://skia.googlecode.com/svn/trunk@12334 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/image.cpp b/gm/image.cpp
index 7ecb604..93e16b7 100644
--- a/gm/image.cpp
+++ b/gm/image.cpp
@@ -186,7 +186,7 @@
         SkAutoTUnref<SkSurface> surf2(SkSurface::NewPicture(info.fWidth, info.fHeight));
         SkAutoTUnref<SkSurface> surf3(SkSurface::NewPicture(info.fWidth, info.fHeight));
 #if SK_SUPPORT_GPU
-        GrContext* ctx = GM::GetGr(canvas);
+        GrContext* ctx = canvas->getGrContext();
 
         SkAutoTUnref<SkSurface> surf4(SkSurface::NewRenderTarget(ctx, info, 0));
 #endif