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.,


Review URL:

git-svn-id: 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/bench.gypi b/bench.gypi
index 0274176..33f0dba 100644
--- a/bench.gypi
+++ b/bench.gypi
@@ -31,6 +31,7 @@
+    '../bench/GrResourceCacheBench.cpp',