Do not make a copy of result in GetBitmapForRenderWidgetHost on Aura.
Do not make a copy of result in GetBitmapForRenderWidgetHost on Aura.

Since the platform independent PlatformCanvas was used on Aura
(http://crrev.com/128354), the reference count issue has not happened any longer.
On the contrary, copying the bitmap retured by PlatformCanvas causes assertion
error in debug build.

BUG=chromium-os:28418
TEST=Manually tested crash does not happen and thumbnail is properly generated


Review URL: http://codereview.chromium.org/9853025

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@129516 0039d316-1c4b-4281-b951-d872f2087c98

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