SystemClipboard: Make use of early return more consistent

Change-Id: Iaed56304098f0db71e0e4c5beb5e9828781e8e59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1656111
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Auto-Submit: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668667}