Remove unnecessary virtual annotation from QuicSpdyStream::priority.

Merge internal change: 186297117

Change-Id: I4f4b9409d2760742cbdae5a71023a7626e0185ae
Reviewed-on: https://chromium-review.googlesource.com/925686


Commit-Queue: Ryan Hamilton <rch@chromium.org>
Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537930}