Add gfx_unittests target to Linux ChromiumOS Tests bots.

BUG=468902
TEST=trybots
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1032543003

Cr-Commit-Position: refs/heads/master@{#322466}

testing/buildbot/chromium.chromiumos.json

@@ -65,6 +65,7 @@
         "test": "sandbox_linux_unittests",
         "args": ["--test-launcher-print-test-stdio=always"]
       },
+      "gfx_unittests",
       "ui_base_unittests",
       "ui_chromeos_unittests",
       "ui_touch_selection_unittests",
@@ -171,6 +172,7 @@
       "skia_unittests",
       "sql_unittests",
       "sync_unit_tests",
+      "gfx_unittests",
       "ui_base_unittests",
       "ui_chromeos_unittests",
       "ui_touch_selection_unittests",
@@ -225,6 +227,7 @@
         "test": "sandbox_linux_unittests",
         "args": ["--test-launcher-print-test-stdio=always"]
       },
+      "gfx_unittests",
       "ui_base_unittests",
       "ui_chromeos_unittests",
       "ui_touch_selection_unittests",