The master protocol description file is js_protocol.pdl, not .json

TBR=yangguo@chromium.org

Change-Id: Ibaef314a63c3e1665ed56bc8c66ca18a00c7fd4f
Reviewed-on: https://chromium-review.googlesource.com/c/1489976Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59882}