Creating a mask layer and painting on it repeatedly exposes texture reuse.

What steps will reproduce the problem?
(1) Apply https://codereview.chromium.org/2125133002
(2) Build with chromeos=1
(3) Run on Linux
(4) Have one browser window open
(5) Press F5 (Overview mode accelerator on Chrome OS) 10 times in about two seconds.

What is the expected output?
with the patch from (1) the browser window in overview is masked with a mask layer that has layer id printout used as a mask. I expect to only ever see one id in the mask.

What do you see instead?
Multiple runs using different mask layers (with different ids) seem to be painting on the same texture.
See attached screenshot. If I wait at least a second between overview mode invocations the mask texture gets cleared.

Bonus points for trying to open multiple (say 10) browser windows and pressing F5. Some of the masked windows in overview will have more than one mask layer ids drawn (second screenshot).

This is a cause of issue 624431 (which is not specific to Files.app).
 

Deleted: mask texture reuse.png 84.5 KB

	mask texture reuse.png 84.5 KB View Download

Deleted: mask texture reuse 2.png 78.2 KB

	mask texture reuse 2.png 78.2 KB View Download