Add gn dependency on rtc_task_runner.

This is step 2 in the process of moving the task_runner abstraction
from webrtc to Chrome. Step 1 was https://codereview.webrtc.org/2696703009/.

BUG=webrtc:6424
TBR=sergeyu@chromium.org

Review-Url: https://codereview.chromium.org/2694363005
Cr-Commit-Position: refs/heads/master@{#452001}
9 files changed