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/image/SkImage.cpp b/src/image/SkImage.cpp
index 795b842..a606384 100644
--- a/src/image/SkImage.cpp
+++ b/src/image/SkImage.cpp
@@ -117,9 +117,8 @@
             return false;
         }
     } else {
-        const SkImageInfo info = SkImageInfo::MakeN32Premul(subset.width(), subset.height());
         SkBitmap tmp;
-        if (!tmp.allocPixels(info)) {
+        if (!tmp.tryAllocN32Pixels(subset.width(), subset.height())) {
             return false;
         }
         *bitmap = tmp;