webkit_layout_tests: Add osmesa as data_dep.

osmesa is a "loadable module" object and hence needs to end up in the
data_deps file.

See content/shell:content_shell_crash_test for another example of this.

BUG=693002,524758
NOTRY=true

Review-Url: https://codereview.chromium.org/2702613003
Cr-Commit-Position: refs/heads/master@{#451226}

diff --git a/BUILD.gn b/BUILD.gn
index e8b26f5..8c08120 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -931,6 +931,7 @@
     testonly = true
     data_deps = [
       "//content/shell:content_shell",
+      "//third_party/mesa:osmesa",
       "//tools/imagediff",
     ]