Some components in the Flutter engine were derived from the forked blink codebase. While the forked components have either been removed or rewritten, the use of the blink namespace has mostly (and inconsistently) remained. This renames the blink namesapce to flutter for consistency. There are no functional changes in this patch.
Also adds //flutter/benchmarking which, similar to //flutter/testing, allows for the creation of a benchmarking executable. This is also the target that contains benchmarking utilities.
