Add osmesa dependency on events_unittests.

BUG=444471

Review URL: https://codereview.chromium.org/812753004

Cr-Commit-Position: refs/heads/master@{#309463}

ui/events/events.gyp

@@ -301,6 +301,7 @@
         '<(DEPTH)/base/base.gyp:test_support_base',
         '<(DEPTH)/skia/skia.gyp:skia',
         '<(DEPTH)/testing/gtest.gyp:gtest',
+        '<(DEPTH)/third_party/mesa/mesa.gyp:osmesa',
         '../gfx/gfx.gyp:gfx',
         '../gfx/gfx.gyp:gfx_geometry',
         '../gfx/gfx.gyp:gfx_test_support',