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/SortBench.cpp b/bench/SortBench.cpp
index 6fb3a71..161e979 100644
--- a/bench/SortBench.cpp
+++ b/bench/SortBench.cpp
@@ -104,10 +104,13 @@
 
 public:
     SortBench(Type t, SortType s) : fType(t), fSortProc(gSorts[s].fProc) {
-        fIsRendering = false;
         fName.printf("sort_%s_%s", gSorts[s].fName, gRec[t].fName);
     }
 
+    virtual bool isSuitableFor(Backend backend) SK_OVERRIDE {
+        return backend == kNonRendering_Backend;
+    }
+
 protected:
     virtual const char* onGetName() SK_OVERRIDE {
         return fName.c_str();