Update getModifierState browser support comments

[nhunzaker]

We have a few stale comments about getEventModifierState:

react/packages/react-dom/src/events/getEventModifierState.js

Lines 24 to 26 in 663835a

	// IE8 does not implement getModifierState so we simply map it to the only
	// modifier keys exposed by the event itself, does not support Lock-keys.
	// Currently, all major browsers except Chrome seems to support Lock-keys.

IE8 isn't an issue anymore, but Safari probably still is. It'd be nice to figure out specifically why we still need this module and update the comment with that info.

[powens]

Mind if I take a look at this?

[nhunzaker]

Not at all :) Thanks!

[powens]

As you mentioned, it appears that the only issue is older versions of Safari (<= 10, according to he caniuse link above). Probably overkill, but I tested a modified create-react-app on Safari 7.1, and the modifierKeyToProp mapping object was used.

It also appears that Chrome now supports Lock-keys. I've removed that note as well.

Let me know if the changes are okay :)