Fix Find behaving like Find Next

This bug happened in two cases when using Find:
1) On macOS, always
2) On all platforms, when text is selected

This change fixes both cases, while also unifying macOS behavior with
other platforms. (I think the previous exclusion of macOS from the
find-from-selection behavior may have been due to a misunderstanding of
some mac-specific code/comments.)

Now, when doing a find-from-selection, the initial result is the same
selected text, rather than the next occurrence. So it's a no-op
(as it should be) in terms of the selection and the document scroll
position.

Bug: 1043550
Change-Id: I78a27b74324f74b1e3c7528fc91badf85f65d22f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145247
Auto-Submit: Russell Davis <russell.davis@gmail.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Jeff Fisher <jeffish@microsoft.com>
Commit-Queue: Jeff Fisher <jeffish@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#759048}

• AUTHORS[diff]
• chrome/browser/ui/find_bar/find_bar_controller.cc[diff]
• chrome/browser/ui/views/find_bar_views_interactive_uitest.cc[diff]