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This CL reimplements to use one CF_HDROP clipboard format having
series of file path's strings instead of series of CF_HDROP having
a file path string to get multiple file paths correctly for
dragging.

BUG=878690

Change-Id: I167e93f6411f866cb143b910b1fc7d4896481ffc
Reviewed-on: https://chromium-review.googlesource.com/1195176Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Miyoung Shin <myid.shin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595759}