make allocPixels throw on failure

BUG=skia:
R=mtklein@google.com, fmalita@google.com, fmalita@chromium.org

Author: reed@google.com

Review URL: https://codereview.chromium.org/510423005
diff --git a/src/utils/SkPDFRasterizer.cpp b/src/utils/SkPDFRasterizer.cpp
index 6663480..1cb792f 100644
--- a/src/utils/SkPDFRasterizer.cpp
+++ b/src/utils/SkPDFRasterizer.cpp
@@ -50,7 +50,7 @@
   char *imgData = image.data();
 
   SkBitmap bitmap;
-  if (!bitmap.allocPixels(SkImageInfo::MakeN32Premul(width, height))) {
+  if (!bitmap.tryAllocN32Pixels(width, height)) {
     return false;
   }
   bitmap.eraseColor(SK_ColorWHITE);