[WebSocket] Rename WebSocketFrame member and update comment.

This is a follow-up patch of previous change:
"[WebSocket] Reduce memcpy at websocket_frame_parser.cc"
https://chromium-review.googlesource.com/c/chromium/src/+/1774420

Bug: 1001915
Change-Id: Ib13c28038a82b4b6ca0eb53a3e4d56ac5242720c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1792030
Reviewed-by: Yutaka Hirano <[email protected]>
Reviewed-by: John Chen <[email protected]>
Reviewed-by: Adam Rice <[email protected]>
Auto-Submit: Yoichi Osato <[email protected]>
Commit-Queue: John Chen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#694826}
13 files changed