I tried repro-ing it locally but was unable to reproduce it so far. I've put up a CL to disable the test on Linux and I'll continue investigating it.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d67dbda9c539acd67efe3fa156c6a139ca54a82f

commit d67dbda9c539acd67efe3fa156c6a139ca54a82f
Author: lfg <lfg@chromium.org>
Date: Wed Mar 29 17:35:33 2017

Disable FlashFullscreenInteractiveBrowserTest.Fullscreen_LockMouse on
Linux.

BUG= 706148 
TBR=scheib@chromium.org,stgao@chromium.org,alexmos@chromium.org

Review-Url: https://codereview.chromium.org/2785553004
Cr-Commit-Position: refs/heads/master@{#460441}

[modify] https://crrev.com/d67dbda9c539acd67efe3fa156c6a139ca54a82f/chrome/browser/ui/exclusive_access/flash_fullscreen_interactive_browsertest.cc

Test is disabled, I'll come back to this issue later.

Marking as available as I'm not sure I'll be able to work on this.

Given that flash is already scheduled for deprecation, and that there's still test coverage on Windows/Mac, I'm not sure if this is worth fixing.

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

Sorry for the inconvenience if the bug really should have been left as Available.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Given that the test still works fine on Windows and Mac, and the flash deprecation timeline, I don't think it's worth fixing the flakyness on Linux. I'll just archive this bug.