In https://codereview.chromium.org/2702613003/ I added //third_party/mesa:osmesa as a data_deps dependency for webkit_layout_tests. This follows the content/shell:content_shell_crash_test target which also does this.

As Dirk mentioned, 
> This doesn't seem right. Why would the layout tests themselves directly need
> this? And isn't this already listed as a data dependency of content_shell?

Need to figure out what is going on here. It is likely got something to do with osmesa being a "loadable module", but I don't really understand GN enough to know how that affects the data deps.