Builds on https://codereview.appspot.com/6718046/ by mtklein.
Previously, each iteration of drawing a picture started new threads to draw the picture. Since each thread is using thread local storage for the font cache, this means that each iteration had to start with an empty font cache.
The newly added MultiCorePictureRenderer, separated from TiledPictureRenderer, now starts the drawing threads at the beginning of the test using an SkThreadPool, and keeps them alive through all iterations, so the font cache can be reused.
For now, I have removed the pipe version of the threaded renderer.
Updated bench_pictures_main and render_pictures_main to use the new
renderer, and to unref a renderer before early exit.
Review URL: https://codereview.appspot.com/6777063
git-svn-id: http://skia.googlecode.com/svn/trunk@6285 2bbb7eff-a529-9590-31e7-b0007b416f81
a62da2fee7