[fsp] Make thread checks consistent.

This patch replaces the old-style check to the new DCHECK_CURRENTLY_ON macro
for consistency.

TEST=Refactoring only. Current tests.
BUG=248427

Review URL: https://codereview.chromium.org/263633003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267782 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed