See e.g.

https://build.chromium.org/p/chromium.webkit/builders/Android%20Builder/builds/97591
https://build.chromium.org/p/chromium.webkit/builders/Android%20Builder/builds/97588
https://build.chromium.org/p/chromium.webkit/builders/Android%20Builder/builds/97567

The second build that we run w/ --ninja-ensure-up-to-date occasionally fails because the first one doesn't seem to stamp the symlink itself, only stamping the input deps:

ninja explain: restat of output image_diff older than most recent input obj/tools/imagediff/imagediff.inputdeps.stamp (0 vs 1479497674)
ninja explain: image_diff is dirty

I suspect this may be related to https://codereview.chromium.org/2485383006/.