use tryAllocPixels to check the result, allocPixels to require success

This is a pending API change in Skia, but call-sites need to stop relying on the return-result of allocPixels first.

Required for chrome CL https://codereview.chromium.org/545513002/

for web/tests
TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/540033002

git-svn-id: svn://svn.chromium.org/blink/trunk@181381 bbb929c8-8fbe-4397-9dbb-9b2b20218538