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/mac/SkCreateCGImageRef.cpp b/src/utils/mac/SkCreateCGImageRef.cpp
index 01b774d..14a24d8 100644
--- a/src/utils/mac/SkCreateCGImageRef.cpp
+++ b/src/utils/mac/SkCreateCGImageRef.cpp
@@ -221,7 +221,7 @@
     int h = (int)CGRectGetHeight(bounds);
 
     SkBitmap bitmap;
-    if (!bitmap.allocPixels(SkImageInfo::MakeN32Premul(w, h))) {
+    if (!bitmap.tryAllocN32Pixels(w, h)) {
         return false;
     }
     bitmap.eraseColor(SK_ColorWHITE);
@@ -287,7 +287,7 @@
     SkImageInfo info = SkImageInfo::MakeN32Premul(width, height);
 
     SkBitmap tmp;
-    if (!tmp.allocPixels(info)) {
+    if (!tmp.tryAllocPixels(info)) {
         return false;
     }