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Out of concern about the compat effects on Android WebView if we change
its UA string override API to prevent custom headers, revert a couple
CHECKs added recently to net::HttpRequestHeaders's setters and to
WebContents's "set the UA override" method. (The consequence is we'll go
back to a state where we're regularly silently handling a DCHECK
failure.)

This reverts:
- f998816 "content: Enforce precondition that UA overrides must..."
- e98b84ec "net: Change HttpRequestHeaders::SetHeader's DCHECKs to CHECK"

Alternatives considered:
- extend the network service API to allow privileged clients to append
arbitrary headers to outgoing requests in order to accommodate the
WebView case, migrating the WebView UA override setter to use this (too
much effort)
- move back to a DCHECK on Android only (saddles //net with platform-
specific logic with a compat motivation far removed from the network
stack)
- keep the CHECKs (could have led to WebView-embedding
apps crashing in production with unknown frequency)

Fixed: 1105745
Change-Id: I4423a68f1884da3a9248041f75c800e4803d4e5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2310774Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790652}