Add can-ignore-rect hint to clear call

src/gpu/gl/GrGpuGL.cpp

Index: src/gpu/gl/GrGpuGL.cpp
===================================================================
--- src/gpu/gl/GrGpuGL.cpp	(revision 12054)
+++ src/gpu/gl/GrGpuGL.cpp	(working copy)
@@ -1255,12 +1255,16 @@
     }
 }
 
-void GrGpuGL::onClear(const SkIRect* rect, GrColor color) {
+void GrGpuGL::onClear(const SkIRect* rect, GrColor color, bool canIgnoreRect) {
     const GrDrawState& drawState = this->getDrawState();
     const GrRenderTarget* rt = drawState.getRenderTarget();
     // parent class should never let us get here with no RT
     SkASSERT(NULL != rt);
 
+    if (canIgnoreRect && this->glCaps().fullClearIsFree()) {
+        rect = NULL;
+    }
+
     SkIRect clippedRect;
     if (NULL != rect) {
         // flushScissor expects rect to be clipped to the target.