Revert "Prepare blink to unify definitions of load completion"

This reverts commit c11d2adcd5204907f2541979ad3c0140756f66de.

Lots of Chromium bots are dying in RSS tests, and this change (which was explicitly called out as a possible troublemaker) was in the changelists when it started. More details at https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/j60j0FXl7mI

TBR=japhet@chromium.org,dcheng@chromium.org,abarth@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=426520

Review URL: https://codereview.chromium.org/679863002

git-svn-id: svn://svn.chromium.org/blink/trunk@184408 bbb929c8-8fbe-4397-9dbb-9b2b20218538