Flex: Draw a border around alignment arrows in the flex overlay
Flex: Draw a border around alignment arrows in the flex overlay

This way, arrows are more easily visible against a variety of background
colors.

Before/After screenshots: https://imgur.com/a/2y1TF29

Bug: 1157856
Change-Id: I8bda1f7dfa40674440e8a42188478dc19488ce77
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2593357
Reviewed-by: Brandon Goddard <37b103671c22e8d219b41940d66d3220ab618ae9@microsoft.com>
Reviewed-by: Robert Paveza <7787129de3d0cb4911c68d958db3023863584560@microsoft.com>
Reviewed-by: Changhao Han <68542faf32f0a558a873c0f094ca9bedb2744bff@chromium.org>
Reviewed-by: Patrick Brosset <f0b1693d3ec18fb190dd6deddb896f34ba3fd693@microsoft.com>
Reviewed-by: Yang Guo <1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org>
Reviewed-by: Mathias Bynens <17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org>
Commit-Queue: Mathias Bynens <17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org>

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