In SkBitmapHeap, defer adding owners for new bitmaps.

In SkBitmapHeap, defer adding owners for new bitmaps.

When using an SkFlatDictionary to flatten shaders, the
dictionary can try to insert a duplicate bitmap shader
that uses a bitmap which has been removed from the
bitmap heap.

This change was originally suggested by junov in
https://codereview.appspot.com/6713048/.

Add a test to verify that deferring the owners works.
Without the change to bitmap heap the test would fail
(and crash in debug mode).

Also remove an unused function from SkFlatDictionary.

BUG=http://code.google.com/p/chromium/issues/detail?id=143923

Review URL: https://codereview.appspot.com/6842051

git-svn-id: http://skia.googlecode.com/svn/trunk@6471 2bbb7eff-a529-9590-31e7-b0007b416f81