Related to tracking java, it would useful to record proguard flags in .size files, so that it's easy to diff them. 

E.g. when we do support library rolls, it would be nice if diagnose_bloat.py would show the proguard flags that changed. 

Reason to add this to .size rather than in diagnose_bloat.py, is that .size files are meant to archive size information, and this is information that's worth archiving IMO.

proguard flags that were used can be found here:
gen/chrome/android/chrome_public_apk/chrome_public_apk.proguard.jar.flags

The format of the file is custom to our build system, so we can change it to be more parsable. May want to strip comments to make it smaller & more diffable.

Probably this information would go well on SizeInfo.proguard_flags, and should be surfaced in Diffs().