DevTools: generate_protocol_externs.js should accept both protocol files as arguments

Since protocol split, the generate_protocol_externs.js CLI has to accept
both protocol files as input.

BUG=none
R=dgozman

Review-Url: https://codereview.chromium.org/2158063002
Cr-Commit-Position: refs/heads/master@{#406052}
1 file changed