Revert "Refactor the locking logic in Resource" (r166704)

This is causing images to disappear from pages when switching tabs.

BUG=342275,342166
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/158733004

git-svn-id: svn://svn.chromium.org/blink/trunk@166822 bbb929c8-8fbe-4397-9dbb-9b2b20218538