Treat VKEY_ASSISTANT as system key

BUG=b/65175944
TEST=locally build and flash, cannot repro the second key tap missing

Change-Id: Ic124a1f9849f5030baef2823908e2bef0d2cf011
Reviewed-on: https://chromium-review.googlesource.com/673268
Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502969}

ash/accelerators/accelerator_router.cc

@@ -22,6 +22,7 @@ namespace {
 // Returns true if |key_code| is a key usually handled directly by the shell.
 bool IsSystemKey(ui::KeyboardCode key_code) {
   switch (key_code) {
+    case ui::VKEY_ASSISTANT:
     case ui::VKEY_MEDIA_LAUNCH_APP2:  // Fullscreen button.
     case ui::VKEY_MEDIA_LAUNCH_APP1:  // Overview button.
     case ui::VKEY_BRIGHTNESS_DOWN: