change asABitmap to isABitmap on shader

BUG=skia:
TBR=

Review URL: https://codereview.chromium.org/1287263005
diff --git a/src/utils/android/SkAndroidSDKCanvas.cpp b/src/utils/android/SkAndroidSDKCanvas.cpp
index b05605b..2eec95f 100644
--- a/src/utils/android/SkAndroidSDKCanvas.cpp
+++ b/src/utils/android/SkAndroidSDKCanvas.cpp
@@ -23,7 +23,7 @@
         return;
     }
 
-    if (shader->asABitmap(NULL, NULL, NULL) == SkShader::kDefault_BitmapType) {
+    if (shader->isABitmap()) {
         return;
     }
     if (shader->asACompose(NULL)) {