Avoid record-time rasterization of gradient patterns.

We already have an uncached path for large tiles:
GeneratorGeneratedImage::drawPatternWithoutCache().

Not only does that fallback avoid record time rasterization, but it turns
out it's actually much faster than drawing into an ImageBuffer and
caching the tile! A modified version of
http://themaninblue.com/experiment/AnimationBenchmark/svg/ with
the following <body> style shows an almost 2x framerate increase:
body
{
  -webkit-border-image: -webkit-repeating-linear-gradient(30deg, black 10%, white 20%) 10 10 10 10 repeat repeat;
}

  * ImageBuffer/cached tile frame rate: ~97fps (http://i.imgur.com/tlES49f.png)
  * native tiling frame rate: ~186fps (http://i.imgur.com/eqgGm8G.png)

This CL rips out the ImageBuffer caching logic and related methods.

About 30 pixel tests require minor rebaselines.

Review URL: https://codereview.chromium.org/27582002

git-svn-id: svn://svn.chromium.org/blink/trunk@159891 bbb929c8-8fbe-4397-9dbb-9b2b20218538