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/src/gpu/vk/GrVkGpu.cpp b/src/gpu/vk/GrVkGpu.cpp
index f7239d6..f34c76c 100644
--- a/src/gpu/vk/GrVkGpu.cpp
+++ b/src/gpu/vk/GrVkGpu.cpp
@@ -1630,8 +1630,8 @@
     return true;
 }
 
-void GrVkGpu::onGetMultisampleSpecs(GrRenderTarget* rt, const GrStencilSettings&,
-                                    int* effectiveSampleCnt, SamplePattern*) {
+void GrVkGpu::onQueryMultisampleSpecs(GrRenderTarget* rt, const GrStencilSettings&,
+                                      int* effectiveSampleCnt, SamplePattern*) {
     // TODO: stub.
     SkASSERT(!this->caps()->sampleLocationsSupport());
     *effectiveSampleCnt = rt->desc().fSampleCnt;