onSavePassword dialog can leak when WebViewClassic is destroyed.

The AlertDialog creates in onSavePassword method leaks if
WebViewClassic is destroyed when the dialog is shown.

Change-Id: I81f20e1dd138467a6413766c0a081b389b334ae0
Improve scrolling cache

Scrolling cache helps make short scrolls/flings smooth but will
cause stutter when long flings are made. This patch disables
scrolling cache when long flings are made.

This patch also fixes a related bug where scrolling cache will
not be enabled properly when transitioning from flinging to scrolling.

Patch Set 2: Calculate threshold based on maximum velocity (Sang Tae Park)

Credits: Pawit Pornkitprasan

Change-Id: I1fdbaad92263c9f983a32900e74eba1f0367902b
Signed-off-by: Arham Jamal <arhamjamal@gmail.com>