Refactor some loops and remove double iteration.

This CL is a minor refactoring to clean up and modernize some code
in webrtcvideoengine.

BUG=None

Review-Url: https://codereview.webrtc.org/3002213002
Cr-Original-Commit-Position: refs/heads/master@{#19660}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0c88a504120738f1d7371bfbf748b17bab7be81d
1 file changed