Google is already pushing WEI (web DRM) into Chromium

This CL moves the base::Feature from content_features.h to
a generated feature from runtime_enabled_features.json5.

This means that the base::Feature can be default-enabled
while the web API is controlled by the RuntimeFeature, which will
still be default-disabled.

An origin trial can enable the RuntimeFeature, which will
allow full access to the API, provided the base::Feature is also
enabled (see change to origin_trial_context.cc).

Meanwhile, the base::Feature can be disabled through Finch as a
kill-switch for the whole feature, and prevent origin trials
from turning the feature on.

Tests have been added to WebView test, as it allowed for easy
spoofing of responses on a known origin.

Bug: 1439945
Change-Id: Ifa0f5d4f5e0a0bf882dd1b0207698dddd6f71420
Fixed: b/278701736
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4681552
Reviewed-by: Rayan Kanso 
Commit-Queue: Peter Pakkenberg 
Reviewed-by: Dmitry Gozman 
Reviewed-by: Richard Coles 
Reviewed-by: Kinuko Yasuda 
Cr-Commit-Position: refs/heads/main@{#1173344}