Rename BrokerFilePermission::unlink_ to temporary_only_

This better describes its function, and avoids confusion later on
should we broker an unlink() call. In that case, the correct permission
check would have nothing to do with the unlink_ member.

Tidy while we're at it:
Remove unused static method (apart from test).
Convert bool functions to return bool expressions.
General readability stuff.

TBR=piman@chromium.org

Change-Id: Id81b9d494a677f8c37865e5145158a03bdb18ae8
Reviewed-on: https://chromium-review.googlesource.com/786083
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519772}

• content/gpu/gpu_sandbox_hook_linux.cc[diff]
• sandbox/linux/syscall_broker/broker_file_permission.cc[diff]
• sandbox/linux/syscall_broker/broker_file_permission.h[diff]
• sandbox/linux/syscall_broker/broker_file_permission_unittest.cc[diff]