DevTools: [CSS] replace CSSProperty.priority with CSSProperty.important

This patch replaces CSSProperty.priority string parameter which is
either unset or "important", with CSSProperty.important boolean parameter
which is much more clear.

BUG=341506
R=apavlov@chromium.org, vsevik@chromium.org

Review URL: https://codereview.chromium.org/178333002

git-svn-id: svn://svn.chromium.org/blink/trunk@167805 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Split ScriptAPIProvider from ServiceWorkerProvider

This 'refactors' the code to make ServiceWorkerProvider
work purely as Scriptable-API-provider rather than
network loading context provider.

Mostly just a small refactoring, plus adding a fixture class
ServiceWorkerContainerClient just to provide ServiceWorkerProvider
instance to ServiceWorkerContainer impl.

BUG=285976

Review URL: https://codereview.chromium.org/163253002

git-svn-id: svn://svn.chromium.org/blink/trunk@167806 bbb929c8-8fbe-4397-9dbb-9b2b20218538