DevTools: fix dark theme Console prompt cutoff
DevTools: fix dark theme Console prompt cutoff

CodeMirror's 'scroll clipping' process re-measures scrollbars upon zoom to
ensure that its artificial scrollbar is hidden. It assumes that the vertical
scrollbar width is equal to horizontal scrollbar height, which is false.

This CL updates dark theme's custom scrollbar height to be the same as its
width.

Screenshot: https://imgur.com/a/eMUQiIt

Bug: 770633
Change-Id: Iddfcca8bc48ee414c0307d6d9e4035e2ed8771f9
Reviewed-on: https://chromium-review.googlesource.com/1083595
Reviewed-by: Dmitry Gozman <0ca14b32cbb359a30ac562af27dd84cc2813eacb@chromium.org>
Commit-Queue: Erik Luo <26192f5e05d23e5ee959014fb911d780e566545b@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#564273}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 36d5b00c49c1adc54accc1e169951cabfda5ddb0
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