__func__ is the c++11 standard replacement for non-standard
__FUNCTION__. See https://chromium-cpp.appspot.com/

This is a mechanical change, done with:

> git grep --name-only __FUNCTION__ \
    | xargs sed -i 's/__FUNCTION__/__func__/g'
> git cl format

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2518403004
Cr-Commit-Position: refs/heads/master@{#436057}