Example failures:
https://ci.chromium.org/buildbot/chromium.mac/Mac10.9%20Tests%20%28dbg%29/49396
https://ci.chromium.org/buildbot/chromium.mac/Mac10.12%20Tests/9664
https://ci.chromium.org/buildbot/chromium.mac/Mac10.10%20Tests/28661

From logs:
[ RUN      ] WebViewPopupInteractiveTest.PopupPositioningMoved
[9728:5891:0130/141643.363760:WARNING:notification_platform_bridge_mac.mm(510)] AlertNotificationService: XPC connection invalidated.
[9728:779:0130/141644.198117:INFO:CONSOLE(6)] "window.runTest: testMoved", source: chrome-extension://impoeoemccngjancadpkcbpabcnomkid/main.js (6)
[9728:779:0130/141644.198697:INFO:CONSOLE(6)] "testMoved", source: chrome-extension://impoeoemccngjancadpkcbpabcnomkid/main.js (6)
[9728:779:0130/141644.526116:INFO:CONSOLE(6)] "webview.loadstop", source: chrome-extension://impoeoemccngjancadpkcbpabcnomkid/main.js (6)
[9728:779:0130/141644.539669:INFO:CONSOLE(20)] "message", source: chrome-extension://impoeoemccngjancadpkcbpabcnomkid/guest.html (20)
[9728:779:0130/141644.541281:INFO:CONSOLE(20)] "data: request-connect", source: chrome-extension://impoeoemccngjancadpkcbpabcnomkid/guest.html (20)
[9728:779:0130/141644.541478:INFO:CONSOLE(6)] "g:message", source: chrome-extension://impoeoemccngjancadpkcbpabcnomkid/main.js (6)
[9728:779:0130/141644.543546:INFO:CONSOLE(6)] "g:data: request-connect", source: chrome-extension://impoeoemccngjancadpkcbpabcnomkid/main.js (6)
2018-01-30 14:16:44.543 Chromium Helper[9737:62294] Couldn't set selectedTextColor from default ()
2018-01-30 14:16:44.544 Chromium Helper[9737:62294] Couldn't set selectedTextBackgroundColor from default ()
[9728:779:0130/141644.545950:INFO:CONSOLE(6)] "message", source: chrome-extension://impoeoemccngjancadpkcbpabcnomkid/main.js (6)
[9728:779:0130/141644.546714:INFO:CONSOLE(6)] "data:response-connect", source: chrome-extension://impoeoemccngjancadpkcbpabcnomkid/main.js (6)
[9728:779:0130/141644.750238:INFO:web_view_interactive_browsertest.cc(491)] DIFF: x = 40, y = 81
../../chrome/browser/apps/guest_view/web_view_interactive_browsertest.cc:501: Failure
Expected: (std::abs(diff.x() - left_spacing)) <= (threshold_px), actual: 20 vs 10
[9728:779:0130/141644.806546:INFO:chrome_cryptauth_service.cc(231)] Profile is not authenticated yet; waiting before starting CryptAuth managers.
[  FAILED  ] WebViewPopupInteractiveTest.PopupPositioningMoved, where TypeParam =  and GetParam() =  (1572 ms)

These CLs showed up around when the test became flaky, but none of them seem relevant. CCing some of the authors, in case they have any insight.

https://chromium.googlesource.com/chromium/src/+/e742243249c3c8cf959356dff9b4a27afeadfc13
https://chromium.googlesource.com/chromium/src/+/7a6683a7924618e96b824f70a8bd0f49c79f4320
https://chromium.googlesource.com/chromium/src/+/65ce0f7c0d647161744517f25015e7308956b0b6
https://chromium.googlesource.com/chromium/src/+/893e1ab42476e1641b2a20fd7a2960b09beffc20
https://chromium.googlesource.com/chromium/src/+/6737e0f2678511ac5c0399d4e26937907fb2648b
https://chromium.googlesource.com/chromium/src/+/ef42564665d915f2d2294d0bfa3a9ab5632c9777
https://chromium.googlesource.com/chromium/src/+/5b205831c0c46f617d23b128e22e1700a9c7d409

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7e46437d4de7278200f2bbe0c6e79dc4702144d4

commit 7e46437d4de7278200f2bbe0c6e79dc4702144d4
Author: Bret Sepulveda <bsep@chromium.org>
Date: Tue Jan 30 23:24:10 2018

Disable flaky WebViewPopupInteractiveTest.PopupPositioningMoved on Mac.

TBR=wjmaclean@chromium.org

Bug: 807446
Change-Id: Icc31a390ab51e2b0819c55cd6336df779ffda2c4
Reviewed-on: https://chromium-review.googlesource.com/894501
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533075}
[modify] https://crrev.com/7e46437d4de7278200f2bbe0c6e79dc4702144d4/chrome/browser/apps/guest_view/web_view_interactive_browsertest.cc

Well, it's disabled. Assigning to a guest view owner to take a look.

Findit says that it is this patch of mine: https://chromium.googlesource.com/chromium/src/+/ef42564665d915f2d2294d0bfa3a9ab5632c9777

Will take a look.