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From https://codepen.io/wesselkroos/pen/KKpGReq, we can clear see the
blur the area modify by the blur effect is actually 3x of the blur
radius. See the screen shot in crbug.com/1117235.

Bug: 1117235
Change-Id: Ib38b70b28bc4bc181ea6366d9d84ad7f7e60b08a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493983Reviewed-by: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: Maggie Chen <magchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821416}