Move some work from backend onClear to base class clear

src/gpu/GrCaps.cpp

Index: src/gpu/GrCaps.cpp
diff --git a/src/gpu/GrCaps.cpp b/src/gpu/GrCaps.cpp
index d6a5470ad52a46a5eb26ce578ae8c31be28ec527..733c55bec4ec973caafb960a9d5d290d6a36a2ba 100644
--- a/src/gpu/GrCaps.cpp
+++ b/src/gpu/GrCaps.cpp
@@ -92,6 +92,7 @@ GrCaps::GrCaps(const GrContextOptions& options) {
     fOversizedStencilSupport = false;
     fTextureBarrierSupport = false;
     fSupportsInstancedDraws = false;
+    fFullClearIsFree = false;
 
     fUseDrawInsteadOfClear = false;
 
@@ -154,6 +155,7 @@ SkString GrCaps::dump() const {
     r.appendf("Oversized Stencil Support          : %s\n", gNY[fOversizedStencilSupport]);
     r.appendf("Texture Barrier Support            : %s\n", gNY[fTextureBarrierSupport]);
     r.appendf("Supports instanced draws           : %s\n", gNY[fSupportsInstancedDraws]);
+    r.appendf("Full screen clear is free          : %s\n", gNY[fFullClearIsFree]);
     r.appendf("Draw Instead of Clear [workaround] : %s\n", gNY[fUseDrawInsteadOfClear]);
     r.appendf("Draw Instead of TexSubImage [workaround] : %s\n",
               gNY[fUseDrawInsteadOfPartialRenderTargetWrite]);