Fix unbound memory consumption problem with run away deferred canvases.

Fix unbound memory consumption problem with run away deferred canvases.

With this CL, deferred canvases will trigger a flush when then the
memory allocated for recording commands (including flattened objects)
exceeds 64MB.

TEST=DeferredCanvas skia unit test, test step TestDeferredCanvasMemoryLimit
BUG=http://code.google.com/p/chromium/issues/detail?id=137884
Review URL: https://codereview.appspot.com/6425053

git-svn-id: http://skia.googlecode.com/svn/trunk@4714 2bbb7eff-a529-9590-31e7-b0007b416f81