Remove Skia legacy API from Blink.

Skia paint effects have recently been refactored to make them  only allocatable
on the heap. This has been achieved by making constructors non-public and
providing factory methods instead.

If necessary, we kept the old API behind a flag so as to not break Chrome or
Blink. This CL updates Blink to use the new API.

BUG=skia:2187

Review URL: https://codereview.chromium.org/236573004

git-svn-id: svn://svn.chromium.org/blink/trunk@171561 bbb929c8-8fbe-4397-9dbb-9b2b20218538