Proof of concept of unity build for WebKit/Source/core/html

Proof of concept of unity build for WebKit/Source/core/html

For testing what effect you get from compiling all of the html
folder in a single compilation unit.

A few more cleanups were necessary but they have already landed as
normal code cleanup on master. The only remaining code change
is to prevent HTMLTokenizer.cpp from removing the
DEFINE_STATIC_LOCAL macro which is also used by other code.

The time change is CPU time going from 25.5 minutes to 1 minute for
a compilation of all files in the folder.