Deprecate webview.use_chromium and introduce webview.force_provider.

The new webview.force_provider property, when set, enforces the use
of Chromium or classic WebView, regardless the value of the
persist.sys.webview.exp (accessible through the settings UI).
This will help telemetry tests to run correctly regardless of UI
settings.

Change-Id: Ica5a0faa7f7539c22332cac5a8946f11fb7df6b2