desks: Make continuous gestures less sensitive.

Altering this number makes a trackpad swipe move around one desk at a
time. This was mostly based off of my own manual testing, and I do not
expect this to be a final value, but in the meantime this unblocks UX
as the previous threshold had the desks moving way too fast for them
to make decisions.

Test: manual
Change-Id: I73ebb2ab3e67fb8b53242698ce7822401fc56d3d
Fixed: 1134738
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490854Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819877}

ash/wm/desks/root_window_desk_switch_animator.h

@@ -192,7 +192,7 @@ class ASH_EXPORT RootWindowDeskSwitchAnimator
 
   // In touchpad units, a touchpad swipe of this length will correspond to a
   // full desk change.
-  static constexpr int kTouchpadSwipeLengthForDeskChange = 100;
+  static constexpr int kTouchpadSwipeLengthForDeskChange = 420;
 
   RootWindowDeskSwitchAnimator(aura::Window* root,
                                int starting_desk_index,