DevTools: force arrows in SoftContextMenu to be white
DevTools: force arrows in SoftContextMenu to be white

Screenshot: https://imgur.com/a/cN62iYX

Bug: 825535
Change-Id: If5ddf68061430539e9a2e69194bb58071bff3bdc
Reviewed-on: https://chromium-review.googlesource.com/1112844
Reviewed-by: Dmitry Gozman <0ca14b32cbb359a30ac562af27dd84cc2813eacb@chromium.org>
Commit-Queue: Erik Luo <26192f5e05d23e5ee959014fb911d780e566545b@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#570218}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7bff7b347b28cf5205d84abbc1a6d2a69af5e78b
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