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Currently, GetElementRect command expects all returned values (x, y,
width, height) to be integers, but in reality they can be non-integer
numbers. This CL fixes the implementation, and improves error messages.

Bug: chromedriver:2545
Change-Id: I8c01f0d7f3de530e5e54b7e698a5583b050b68a3
Reviewed-on: https://chromium-review.googlesource.com/1178759Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583851}