Reland "win: tweaks to touch drag."

This is a reland of commit 69eb90b5
with a fix for the test failure, which is to move the setting
of drag_drop_in_progress_ so that it's set for both the touch
and non-touch case.

Original change's description:
> win: tweaks to touch drag.
>
> Verify that mouse cursor is in the drag source window.
>
> Tested manually.
>
> Low-Coverage-Reason: HARD_TO_TEST
> Bug: 40312079, 370856871
> Change-Id: I800e915f566d63354e4e1d1e1f7edd79200f7553
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5927282


> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1370869}

(cherry picked from commit 5398c8d0)

Low-Coverage-Reason: HARD_TO_TEST
Bug: 40312079, 370856871, 374385590
Change-Id: I387757524615c3b3fe4dd6865825331db5bee44e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5943487


Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1371525}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5952499


Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: David Bienvenu <davidbienvenu@google.com>
Cr-Commit-Position: refs/branch-heads/6778@{#781}
Cr-Branched-From: b21671ca-refs/heads/main@{#1368529}