Save, then restore scroll position in Chrome textareas on remarkup assist

Summary:
Fixes T10396. Seems like this has been around for a while (references from 2011):

http://stackoverflow.com/questions/4002312/chrome-resets-the-textarea-scroll-bar-scrolltop-when-focus-is-called
https://bugs.chromium.org/p/chromium/issues/detail?id=75072

Commenting out this focus() seemed to fix the issue locally, at the cost of not focusing.

Saving, focusing, then restoring seems to produce the correct behavior everywhere.

Test Plan:

• In Safari, Firefox and Chrome, typed a ton of text into a remarkup area (more than the height of the area, so it has a scrollbar).
• Selected some text near the top.
• Clicked "B" to bold the text.
• Scroll position remained the same in all browsers (previously: in Chrome, it changed).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10396

Differential Revision: https://secure.phabricator.com/D15313