Move memoization of multisample specs id to GrRenderTarget

GrGpu used to perform the memoization on behalf of GrRenderTarget via
a public accessor. This change updates GrRenderTarget to do its own
work.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2467593002

Review-Url: https://codereview.chromium.org/2467593002
diff --git a/tools/gpu/GrTest.cpp b/tools/gpu/GrTest.cpp
index 90751f0..ba870ea 100644
--- a/tools/gpu/GrTest.cpp
+++ b/tools/gpu/GrTest.cpp
@@ -303,8 +303,8 @@
                        const SkIRect& srcRect,
                        const SkIPoint& dstPoint) override { return false; }
 
-    void onGetMultisampleSpecs(GrRenderTarget* rt, const GrStencilSettings&,
-                               int* effectiveSampleCnt, SamplePattern*) override {
+    void onQueryMultisampleSpecs(GrRenderTarget* rt, const GrStencilSettings&,
+                                 int* effectiveSampleCnt, SamplePattern*) override {
         *effectiveSampleCnt = rt->desc().fSampleCnt;
     }