Desks: Put the touchpad gestures behind a flag

There're still improvements we want for Virtual Desks gestures.
This CL puts the touchpad gestures behind a flag until we can
land the full experience.

This means users who want to enable and use touchpad gestures
to switch virtual desks, will have to enable the flag
"--enable-virtual-desks-gestures" from chrome://flags. The flag
will be disabled by default.

* Flag Disabled:
    - 3-finger horizontal swipes -> Tab scrubbing and overview highlighter.
    - Touchpad gestures to switch desks will be disabled.

* Flag Enabled:
    - 3-finger horizontal swipes -> Switch desks.
    - 4-finger horizontal swipes -> Tab scrubbing & overview highlighter.

BUG=1005340
TEST=Manually, and modified tests.

Change-Id: If867b444a62d0c5853fba4b21b8d74125dfe972b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1811802
Commit-Queue: Ahmed Fakhry <[email protected]>
Reviewed-by: James Cook <[email protected]>
Cr-Commit-Position: refs/heads/master@{#698116}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index e48e277..c174248 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1729,6 +1729,10 @@
     {"enable-virtual-desks", flag_descriptions::kEnableVirtualDesksName,
      flag_descriptions::kEnableVirtualDesksDescription, kOsCrOS,
      FEATURE_VALUE_TYPE(ash::features::kVirtualDesks)},
+    {"enable-virtual-desks-gestures",
+     flag_descriptions::kEnableVirtualDesksGesturesName,
+     flag_descriptions::kEnableVirtualDesksGesturesDescription, kOsCrOS,
+     FEATURE_VALUE_TYPE(ash::features::kVirtualDesksGestures)},
     {"trim-on-all-frames-frozen", flag_descriptions::kTrimOnFreezeName,
      flag_descriptions::kTrimOnFreezeDescription, kOsCrOS,
      FEATURE_VALUE_TYPE(performance_manager::features::kTrimOnFreeze)},