mac: Add a hack to prevent a BookmarkButton dragging crash from triggering.
mac: Add a hack to prevent a BookmarkButton dragging crash from triggering.

The root of the problem lies in the fact that -[draggableButtonImpl_
mouseDownImpl:theEvent] spins a run loop, and by the time the function returns,
the original -mouseDown: event may no longer be relevant.

BUG=509833

Review URL: https://codereview.chromium.org/1296203004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#343968}

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