Clean up exported PRs after chromium CLs are abandoned

We now run this as a step in wpt exporter. After a chromium CL
has been abandoned for more than two weeks, the exported PR will
be closed automatically.

Merged CL with no exportable changes will cause a previously exported
Pr to be closed immediately.

Bug: 1034348
Change-Id: I835acb1ef98f5eab0fd9776eb99da13533ff1c47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4049884


Auto-Submit: Weizhong Xia <weizhong@google.com>
Reviewed-by: Jonathan Lee <jonathanjlee@google.com>
Commit-Queue: Weizhong Xia <weizhong@google.com>
Cr-Commit-Position: refs/heads/main@{#1075423}