Report first_paint in milliseconds
Report first_paint in milliseconds

firstPaintTime is reported in seconds.microseconds.  All other times are
reported in milliseconds with optional microsecond precision.

This patch changes first_paint from seconds.microseconds to
milliseconds.microseconds with no loss of precision.

forcedotcom/kylie,sdgdsffdsfff/boomerang,levexis/boomerang,levexis/boomerang,lognormal/boomerang,sdgdsffdsfff/boomerang,forcedotcom/kylie,FredXue/boomerang,FredXue/boomerang,lognormal/boomerang