Web Inspector: RTL - DOM Tree Element selection doesn't work

Web Inspector: RTL - DOM Tree Element selection doesn't work
https://bugs.webkit.org/show_bug.cgi?id=181275
<rdar://problem/36290450>

Patch by Joseph Pecoraro <pecoraro@apple.com> on 2018-01-03
Reviewed by Devin Rousso.

* UserInterface/Views/TreeOutline.js:
(WI.TreeOutline.prototype.treeElementFromEvent):
Provide a better explanation for why we are making the `x` adjustment here,
to detect the inner most tree element along the horizontal. Fix the algorithm
for RTL, since the intent is to adjust to the trailing edge of the container
which is on the opposite side in RTL.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@226392 268f45cc-cd09-0410-ab3c-d52691b4dbfc