servo: add a stub for mistral

BUG=none
BRANCH=none
TEST=servod -b mistral works, but some functions are still broken

Change-Id: I592a3f8abdd131c0202f62d564bd18d38398aaa3
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1313488
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-by: Zhihong Yu <zhihongyu@chromium.org>
diff --git a/servo/data/servo_mistral_overlay.xml b/servo/data/servo_mistral_overlay.xml
new file mode 100644
index 0000000..093d936
--- /dev/null
+++ b/servo/data/servo_mistral_overlay.xml
@@ -0,0 +1,2 @@
+<root>
+</root>