Remove WebSocketHandshakeReuseConnection flag and feature

Remove the feature and flag for WebSocketHandshakeReuseConnection. It is
now unconditionally enabled.

After this, the flag no longer appears in about:flags.

BUG=423609

Change-Id: Iefd66092f99654f86a9e61b1fbe7fde8a8665164
Reviewed-on: https://chromium-review.googlesource.com/c/1358312
Commit-Queue: Adam Rice <ricea@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621576}
6 files changed