Move OneShotEvent into base/.

This CL moves the OneShotEvent primitive into base/, as it
now satisfies the rule of 3 (chromecast/, extensions/, chrome/).

[email protected], [email protected]

Bug: None
Change-Id: I52bea9524ee1f451d8c76d01869cbb382a738446
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1522498
Commit-Queue: calamity <[email protected]>
Reviewed-by: calamity <[email protected]>
Reviewed-by: danakj <[email protected]>
Cr-Commit-Position: refs/heads/master@{#647610}
43 files changed