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Focus gain causes virtual keyboard to show. Then virtual keyboard takes
the bottom inset and causes the window and view size to shrink, on
chrome and sometimes on webview.
At this point, the bottom inset may occlude the input form,
i.e. the input form is behind the keyboard and is not visible.

We have the following auto-scrolling logic to mitigate the inconvenience
here:
1) Check the size of visible display size after showing keyboard.
2) Wait for the view height change after showing keyboard.
3) Change the viewport size
4) Scroll focused editable node into view.

In the browser, this is in order. However, in the renderer, the order
of 3) and 4) sometimes gets reversed.
I suspect that this happens because the two messages are going through
different IPC channels.

Now we do the following:
1) Wait until SynchronizeVisualProperties() changes visual viewport.
2) Detect that blink's scrollable viewport size has changed
3) Comfirm this signal against the visible display size
4) Scroll focused editable node into view

This also adds some test and refactors ImeAdapterImpl.

Bug: 920061

Change-Id: Id237fa76c304c92b5459bda4fe6591c96cb804d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1562585Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Changwan Ryu <changwan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682991}