Fix compilation after https://codereview.chromium.org/375183002

https://codereview.chromium.org/375183002 added a new pure virtual method
IsTranslucentWindowOpacitySupported. We need to at least provide
an empty implementation.

Note that this patch was backported in the M37 branch with
https://codereview.chromium.org/412033002 thus it's needed for whoever
moves to latest version of M37 (note that this patch was not present
in 37.0.2062.20).
Merge pull request #263 from darktears/Milestone-Summer

Fix compilation after https://codereview.chromium.org/375183002