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When file loading is finished ResourceMultiBufferDataProvider count
DataBuffer queue size twice, so GetSize returned incorrect value.

BUG=625515, 627525, 624853

patch from issue 2110853006 at patchset 1 (http://crrev.com/2110853006#ps1)

Review-Url: https://codereview.chromium.org/2133803004
Cr-Commit-Position: refs/heads/master@{#405242}