WebView: Add the definition of the renderer importance API.

The renderer importance API is used to specify how important
out-of-process WebView renderer services are for the purposes of OOM
killing and scheduling with respect to the binding application.

This allows an application to - for example - specify that renderers can
be killed while the application is not in the foreground, thus cleaning
up additional resources.

Bug: 30824898
Test: Tests await Chromium change.
Change-Id: I6dca3d427d6cdb5cb7e0be6f7fb8ece64bd24af9