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/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index ce35ba2..261640f 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -343,7 +343,7 @@
         return false;
     }
 
-    if (!dst->allocPixels(src.info())) {
+    if (!dst->tryAllocPixels(src.info())) {
         return false;
     }