webkit_tests failing on chromium.webkit/WebKit Mac10.9

Builders failed on: 
- WebKit Mac10.9: 
  https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Mac10.9


Total tests: 69800
* Passed: 57157 (57080 expected, 77 unexpected)
* Skipped: 11818 (11818 expected, 0 unexpected)
* Failed: 781 (780 expected, >>>1 unexpected<<<)
* Flaky: 44 (44 expected, 0 unexpected)

Unexpected Failures:
* fast/text/variable-fonts/variable-gpos-m2b.html

First happened in build https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Mac10.9/builds/45664

Problematic commit: https://chromium.googlesource.com/chromium/src/+/0b70e87ed73d1b64b4ba9acbc4615b6fba7094f7
Test Variations GPOS mark to base placement

Add a reftest using an additional axis VM2B in the variabletest_box
font. The VM2B axis shifts the mark anchor to the left, leading to the
combining mark below box shifting right when the axis parameter is
adjusted from 0 to 1000. As the reference in the ref test, the font now
contains two additional glyphs N and O, which have fixed base anchors at
the middle and right position. A rendering of the M character showing
"m2b pos", followed by COMBINING SQUARE BELOW with VM2B 0, 500, 1000
applied must thus be equal to renderings of the N and O glyph with no
VM2B variation values applied.

Dominik: if you can't find a quick fix can you please revert the CL while you investigate?  Thanks.