<chromium> Disable warnings as errors on Mac and Windows

Since we have little chance of introducing warnings in Chromium code
and that we allow building the code on more various toolchains,
avoid breaking the build for warnings.

We already disable them on os_posix==1 and OS!="mac" by overriding
the werror% gyp variable in qtwebengine_extras.gypi.

Change-Id: Ic233bcd018a9a78a508772dea126435e2226b721
Reviewed-by: Andras Becsi <andras.becsi@digia.com>