Adds new configuration property to specify minimum fragment size.

<set-configuration-property name="compiler.splitpoint.leftovermerge.size" value="size in bytes"/>

Any exclusive fragment smaller than this limit will be merged into the left overs fragment.

Change-Id: Ifcf035d88db70e890c739eab3997b85fa2e5677e
Review-Link: https://gwt-review.googlesource.com/#/c/2581/