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/tests/DrawBitmapRectTest.cpp b/tests/DrawBitmapRectTest.cpp
index 6dca98b..720155c 100644
--- a/tests/DrawBitmapRectTest.cpp
+++ b/tests/DrawBitmapRectTest.cpp
@@ -190,7 +190,9 @@
     c.concat(matrix);
 
     SkBitmap bm;
-    bm.allocN32Pixels(width, height);
+    if (bm.tryAllocN32Pixels(width, height)) {
+        // allow this to fail silently, to test the code downstream
+    }
     bm.eraseColor(SK_ColorRED);
 
     matrix.setAll(0.0078740157f,