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The frame shouldn't be modified by the aggregator, or else it will be broken if GetEligibleFrame is called again.
Also modify ScopedPtrVector::const_iterator to make it const-correct.

BUG=

Review URL: https://codereview.chromium.org/417843003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285431 0039d316-1c4b-4281-b951-d872f2087c98