DevTools: [CSS] fix style ranges updating

DevTools CSSOM objects (like WebInspector.CSSRule,
WebInspector.CSSStyleDeclaration) are addressed by their ranges in source code
(aka "source ranges"). Whenever some property gets changed, all the
source ranges that lay afterwards in the same stylesheet have to be updated.

Unfortunately, there was a typo in this bookkeeping operation. This patch
fixes it and covers with a testcase.

BUG=390300
R=apavlov, vsevik

Review URL: https://codereview.chromium.org/368013002

git-svn-id: svn://svn.chromium.org/blink/trunk@177401 bbb929c8-8fbe-4397-9dbb-9b2b20218538