Call WebContentsDelegate::CanDragEnter from WebContentsViewAura.

Call WebContentsDelegate::CanDragEnter from WebContentsViewAura. Allows applications using the Content API to selectively cancel (disallow) drag&drop operations on a per-WebContents basis. For example, applications may want to disable drag&drop completely or may want to disallow a drop based on the drag contents (only certain data types, URL formats, etc).

BUG=224440
TEST=none

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=269664

[Marshall, 2014-05-06 20:49:52]

Please review.

[Ben Goodger (Google), 2014-05-09 22:11:02]

lgtm

[Marshall, 2014-05-09 22:58:58]

The CQ bit was checked by marshall@chromium.org

[commit-bot: I haz the power, 2014-05-09 23:01:41]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/marshall@chromium.org/267113008/1

[commit-bot: I haz the power, 2014-05-10 02:56:16]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  linux_chromium_clang_dbg on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_clang_...)

[commit-bot: I haz the power, 2014-05-11 00:57:03]

Change committed as 269664