DevTools: [SSP] show source location for the newly inserted style rules.

To achieve the specified goal, the patch does the following:
- StylePropertiesSection.styleRule no longer has sourceURL as it could
be always fetched from styleRule.rule
- StylePropertiesSection._createRuleOrigin is receiving CSSRule and
ruleLocation as arguments
- BlankStylePropertiesSection is taught to infer the actual rule
location of the rule that will be inserted

Layout Test change: the CSSOM rule now don't show source location (used to be "located" on first line)

R=pfeldman, vsevik, apavlov

Review URL: https://codereview.chromium.org/474433004

git-svn-id: svn://svn.chromium.org/blink/trunk@180348 bbb929c8-8fbe-4397-9dbb-9b2b20218538