Store the GrBackend we are using on GrContext

As we start adding more functionality to external clients and passing
data around, we will need to know what backend we are working with at
a layer higher that GrGpu.

TBR=bsalomon@google.com

Bug: skia:
Change-Id: Ifee527ea0c1046469401a39e0500f84a46bb3081
Reviewed-on: https://skia-review.googlesource.com/13275
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>

include/gpu/GrContext.h

@@ -334,6 +334,8 @@ private:
 
     GrAuditTrail                            fAuditTrail;
 
+    GrBackend                               fBackend;
+
     // TODO: have the GrClipStackClip use renderTargetContexts and rm this friending
     friend class GrContextPriv;
 

src/gpu/GrContext.cpp

@@ -83,6 +83,8 @@ bool GrContext::init(GrBackend backend, GrBackendContext backendContext,
     ASSERT_SINGLE_OWNER
     SkASSERT(!fGpu);
 
+    fBackend = backend;
+
     fGpu = GrGpu::Create(backend, backendContext, options, this);
     if (!fGpu) {
         return false;

src/gpu/GrContextPriv.h

@@ -157,6 +157,8 @@ public:
                             size_t rowBytes,
                             uint32_t pixelOpsFlags = 0);
 
+    GrBackend getBackend() const { return fContext->fBackend; }
+
 private:
     explicit GrContextPriv(GrContext* context) : fContext(context) {}
     GrContextPriv(const GrContextPriv&); // unimpl