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Fix the rounding error in code for coordinate conversion from
native host coordinate to screen DIP coordinate.

In the original code, when mouse cursor is in the warp region of
secondary display, its screen DIP coordinate which is computed by
ConvertHostPointToRelativeToRootWindow may be out of secondary display
due to rounding error. It is also found that this rounding error only
happens with specific display zoom factors and display rotation
degrees.

test: ash_unittests

Bug: 905035
Change-Id: I36af26afe4e210d70b6812a1bddb1a95feb7235f
Reviewed-on: https://chromium-review.googlesource.com/c/1343249Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612923}