Add WriteHTMLForTraceDataToFile method.

Add WriteHTMLForTraceDataToFile method that allows creating trace viewer in html directly from trace data. 
Sure. However, when I am making the change and test it, I find out that something is broken at google:master. Generating html trace for multiple trace files produces a bogus view of the traces.
You can reproduce the bug by:
./trace2html test_data/huge_trace.json test_data/big_trace.json --output combine_trace.html
![screen shot 2014-10-23 at 8 59 02 am](https://cloud.githubusercontent.com/assets/911877/4756463/8d8b421e-5acd-11e4-910d-0d967b018bd2.png)