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This CL adds event handling for XRandR |XEvent|s, re-fetching and updating
display list dynamically and communicating |DisplayObserver|s about the
changes.

No unit tests are added due to the current dependency on XRandR/Xlib API.

Bug: 891175
Test: None
Change-Id: I96c5fe83fc4f1792503b34b0676b778b601d42d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1709491
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680591}