Refactoring how SkDeferredCanvas manages mutable bitmaps

Refactoring how SkDeferredCanvas manages mutable bitmaps

This CL makes the SkGPipe flavor of SkDeferredCanvas properly
decide whether to flush or record mutable bitmaps.  The flushing
is now managed by conditionally switching the canvas to non-deferred
mode, which avoids an unnecessary transient copy of the bitmap.

BUG=http://code.google.com/p/chromium/issues/detail?id=137884
TEST=DeferredCanvas unit test, sub test TestDeferredCanvasMemoryLimit
Review URL: https://codereview.appspot.com/6421060

git-svn-id: http://skia.googlecode.com/svn/trunk@4756 2bbb7eff-a529-9590-31e7-b0007b416f81