Add bootstrap performance measuring support (#11188)

svn changeset:25664/svn branch:6.8

Conflicts:
	client/src/com/vaadin/client/ApplicationConnection.java
	uitest/src/com/vaadin/tests/performance/BasicPerformanceTest.java
	uitest/src/com/vaadin/tests/util/TestUtils.java

Change-Id: I699e7b47ad5a62b67dbdf1004da5e5daf009ba25