Add check in GrGpu createTexture to make sure we don't create multisampled textures.

BUG=skia:

Review URL: https://codereview.chromium.org/1138223003
diff --git a/src/gpu/GrGpu.cpp b/src/gpu/GrGpu.cpp
index 15e4b28..b7d362c 100644
--- a/src/gpu/GrGpu.cpp
+++ b/src/gpu/GrGpu.cpp
@@ -76,6 +76,11 @@
         return NULL;
     }
 
+    // We currently not support multisampled textures
+    if (!isRT && desc.fSampleCnt > 0) {
+        return NULL;
+    }
+
     GrTexture *tex = NULL;
 
     if (isRT) {