tap2, tap3, tap4 enums were missing

These gesture enums were missing.  Prevented two finger tap
from silencing the screen reader.

Bug: 888695
Test: Manual
Change-Id: I6e71cf65de19651ea3560fbf825cfe59ca6cdcbb
Reviewed-on: https://chromium-review.googlesource.com/1241274Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595111}

chromecast/common/extensions_api/accessibility_private.json

@@ -35,7 +35,7 @@
       {
         "id": "Gesture",
         "type": "string",
-        "enum": [ "click", "swipeLeft1", "swipeUp1", "swipeRight1", "swipeDown1", "swipeLeft2", "swipeUp2", "swipeRight2", "swipeDown2", "swipeLeft3", "swipeUp3", "swipeRight3", "swipeDown3", "swipeLeft4", "swipeUp4", "swipeRight4", "swipeDown4" ],
+        "enum": [ "click", "swipeLeft1", "swipeUp1", "swipeRight1", "swipeDown1", "swipeLeft2", "swipeUp2", "swipeRight2", "swipeDown2", "swipeLeft3", "swipeUp3", "swipeRight3", "swipeDown3", "swipeLeft4", "swipeUp4", "swipeRight4", "swipeDown4", "tap2", "tap3", "tap4" ],
         "description": "Accessibility gestures fired by the touch exploration controller."
       },
       {