Bumped the opacity value to 12% for hover and 16% for pressed. This only applies for incognito. 

Preview attached. Full spec updated: https://drive.google.com/a/google.com/file/d/0B6x6iYCtKinEVzZma1BDT3ZuaUU/view?usp=sharing

Deleted: hover.png 16.0 KB

	hover.png 16.0 KB View Download

Deleted: pressed.png 10.5 KB

	pressed.png 10.5 KB View Download

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/45e5facee3ebc508b5c7c59d9c9940d7eac53af3

commit 45e5facee3ebc508b5c7c59d9c9940d7eac53af3
Author: shrike <shrike@chromium.org>
Date: Wed Apr 06 22:30:47 2016

[Mac] Increase contrast of toolbar button hover bg in MD Incognito.

The toolbar button hover states are difficult to see in Material Design
Incognito mode. This change increases the contrast.

R=avi@chromium.org
BUG= 593371 

Review URL: https://codereview.chromium.org/1858393002

Cr-Commit-Position: refs/heads/master@{#385570}

[modify] https://crrev.com/45e5facee3ebc508b5c7c59d9c9940d7eac53af3/chrome/browser/ui/cocoa/toolbar/toolbar_button_cocoa.mm