Defer flushes if kPreferNoIO is specified

Prior to this patch clients who were solely uploading to textures (e.g., SW Mask Mgr) would cause extra flushes b.c., even though kPreferNoIO was being specified, resources with pending IO would still be returned even though there was plenty of space in the resource cache.

Review URL: https://codereview.chromium.org/1286203002
diff --git a/src/gpu/GrGpu.cpp b/src/gpu/GrGpu.cpp
index 9c468e7..8c58321 100644
--- a/src/gpu/GrGpu.cpp
+++ b/src/gpu/GrGpu.cpp
@@ -79,7 +79,7 @@
         return NULL;
     }
 
-    // We currently not support multisampled textures
+    // We currently do not support multisampled textures
     if (!isRT && desc.fSampleCnt > 0) {
         return NULL;
     }