Revert of Remove remnants of DHE support. (patchset #3 id:40001 of https://codereview.chromium.org/2653773003/ )

Reason for revert:
I suspect this breaks https://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%281%29/builds/62851 since the trybot that ran (and passed) was a Win x64 config (win_chromium_x64_rel_ng).
The x64 windows bot doesn't break post-commit.
This was the only CL in the blame list touching encryption IIUC.

Original issue's description:
> Remove remnants of DHE support.
>
> The admin policy has now expired, so we can unwind all code relating to
> it.
>
> The deprecated cipher fallback will be removed in a follow-up, tracked
> by https://crbug.com/684730.
>
> BUG=619194
>
> Review-Url: https://codereview.chromium.org/2653773003
> Cr-Commit-Position: refs/heads/master@{#446049}
> Committed: https://chromium.googlesource.com/chromium/src/+/6b4943bfc61acc877f92b183b849922edbe85e8d

TBR=svaldez@chromium.org,pastarmovj@chromium.org,davidben@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=619194

Review-Url: https://codereview.chromium.org/2656953002
Cr-Commit-Position: refs/heads/master@{#446249}
11 files changed