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/bench/WriterBench.cpp b/bench/WriterBench.cpp
index 0956b22..f9a0ac8 100644
--- a/bench/WriterBench.cpp
+++ b/bench/WriterBench.cpp
@@ -12,7 +12,9 @@
 
 class WriterBench : public SkBenchmark {
 public:
-    WriterBench() { fIsRendering = false; }
+    virtual bool isSuitableFor(Backend backend) SK_OVERRIDE {
+        return backend == kNonRendering_Backend;
+    }
 
 protected:
     virtual const char* onGetName() SK_OVERRIDE {