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This adds the code necessary to highlight flex containers on hover in
DevTools.
For now, only the container's content box can be highlighted.

Frontend CL: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2488661/

Bug: 1139949
Change-Id: I8268227095bec552f45cc93d4240b555c3ad2534
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487065
Commit-Queue: Patrick Brosset <patrick.brosset@microsoft.com>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824351}