Add a webApiUsage property to control whether prefixed API's will be used.

This is a simple implementation that doesn't include browser or
feature-specific settings.

Change-Id: I416794f99bafa78a9a0ef84ef698d372c8263613
Review-Link: https://gwt-review.googlesource.com/#/c/1840/

Review by: goktug@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11495 8db76d5a-ed1c-0410-87a9-c151d255dfc7