blink_platform_unittests failing on chromium.webkit/WebKit Mac10.11 (retina)

Builders failed on: 
- WebKit Mac10.11 (retina): 
  https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Mac10.11%20%28retina%29

See discussion on https://chromium-review.googlesource.com/c/chromium/src/+/749221

Test was disabled on https://chromium-review.googlesource.com/786373 so this should cycle green.

(Note: Please file a separate bug for failing tests, separate to the bug for implementing the feature that caused the failure. It's very hard for the sheriff to confirm that action has been taken when SOM links to a non-closed bug to implement a feature. Once the test has been disabled, you close the failing-test bug as fixed, and then sheriff can see that action has been taken but the CQ hasn't cycled green.)