Fixing use after free error in Canvas2DLayerBridge due to vector reallocation

The released MailboxInfo object needs to be referenced by array index
rather than by pointer, in case the m_mailboxes vector is reallocated.

BUG=339610

Review URL: https://codereview.chromium.org/150903002

git-svn-id: svn://svn.chromium.org/blink/trunk@166334 bbb929c8-8fbe-4397-9dbb-9b2b20218538