Blink: Set access policy to protected mode in Drag Drop events

As per spec
https://html.spec.whatwg.org/multipage/dnd.html#concept-dnd-p
The correct access restriction for most drag events
after "dragstart" is the protected mode.
which allows enumerating the types/kind of data in the drag store.

DataTransferAccessPolicy::kImageWritable is
no longer in use and is removed.

Bug: 1345474
Change-Id: Ibcbe4f61f5846ba49b590d235324f2784370d005
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5341049


Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Rakesh Goulikar <ragoulik@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1279266}