4th side of a 3-sided patch

Remove the now unnecessary defines introduced in
https://codereview.chromium.org/348193002

R=abarth@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/355563002

git-svn-id: svn://svn.chromium.org/blink/trunk@177217 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Expose 'LaxMixedContentChecking' runtime setting to embedders.

Apparently WebRuntimeSettings isn't generated. Who knew? This followup
to https://codereview.chromium.org/356843006/ makes it possible for the
embedder to set this flag, which we'll need to do if we want enterprise
policy to hook into the setting.

BUG=389089

Review URL: https://codereview.chromium.org/359183003

git-svn-id: svn://svn.chromium.org/blink/trunk@177218 bbb929c8-8fbe-4397-9dbb-9b2b20218538