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/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 34d1b97..e8d6d99 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -999,7 +999,7 @@
 
 static SkBitmap capture_bitmap(SkCanvas* canvas) {
     SkBitmap bm;
-    if (bm.allocPixels(canvas->imageInfo())) {
+    if (bm.tryAllocPixels(canvas->imageInfo())) {
         canvas->readPixels(&bm, 0, 0);
     }
     return bm;