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This CL adds back the source bit on injected ACTION_SCROLL motion
events.

This got lost when [1] tried to make this bit a parameter and then
subsequently removed the option due to performance regressions in [2].

The reason this breaks is because in ContentUiEventHandler.java,
onGenericMotionEvent only dispatches a wheel event if the ACTION_SCROLL
event comes from a Pointer source[3].

[1] https://crrev.com/02cd150c24de8bdebb14d88c1a144b921a47756f
[2] https://crrev.com/14e88d0ddbdd58d346bc9c3bb0629af3fdf56f0f
[3] https://cs.chromium.org/chromium/src/content/public/android/java/src/org/chromium/content/browser/ContentUiEventHandler.java?rcl=3df6abb639be7564b7743ade8db455a3564e7d32&l=67

Change-Id: I1ba4143f61ad889173735b0a168d7790a00d0865
Bug: 1025789
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1935432Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Auto-Submit: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719233}