DevTools: cleanup InspectorStyleSheet

This patch cleans up InspectorStyleSheet after the InspectorStyleSheet
and InspectorStyleSheetForInlineStyle implementations were separated.
It does the following changes:
- remove InspectorStyleSheet.checkPageStyleSheet as it is never used
- inline InspectorStyleSheet.ruleIndexByRule
- inline method InspectorStyleSheet.ruleIdByStyle into InspectorStyleSheet.styleId
- introduce single implementation for lineEndings in InspectorStyleSheetBase
- eliminate method InspectorStyleSheetBase.reparsestyleSheet

BUG=341506

Review URL: https://codereview.chromium.org/203633003

git-svn-id: svn://svn.chromium.org/blink/trunk@169574 bbb929c8-8fbe-4397-9dbb-9b2b20218538