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/gpu/GrSurface.cpp b/src/gpu/GrSurface.cpp
index a07fe67..54497fe 100644
--- a/src/gpu/GrSurface.cpp
+++ b/src/gpu/GrSurface.cpp
@@ -25,8 +25,7 @@
 
 bool GrSurface::savePixels(const char* filename) {
     SkBitmap bm;
-    if (!bm.allocPixels(SkImageInfo::MakeN32Premul(this->width(),
-                                                   this->height()))) {
+    if (!bm.tryAllocPixels(SkImageInfo::MakeN32Premul(this->width(), this->height()))) {
         return false;
     }