Forward declare WTF::CrossThreadOnceFunction and WTF::CrossThreadOnceClosure

... similarly to their bind repeating couter parts (namely
WTF::CrossThreadFunction and WTF::CrossThreadClosure).

This way callers can call both functions directly, without the explicit
WTF namespace

BUG=963574
R=haraken@chromium.org, hiroshige@chromium.org

Change-Id: I8385b2765ee9502abf2d28437f27e007e9218feb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619839
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#661445}
1 file changed