[omnibox] Constrain 'IsSelectAll()' to be more accurate

Currently, 'IsSelectAll()' returns true for a range of (0,0) but the
text is empty.  This isn't correct, and a range with identical
endpoints actually represents a cursor/caret position with nothing
selected.  Specifically, (0,0) is what the Omnibox code requests to
'home' the cursor.  Returning true is causing a problem for
'OmniboxViewViews::Update()' which then thinks it should select all of
the new text.


Review-Url: https://codereview.chromium.org/2942013002
Cr-Commit-Position: refs/heads/master@{#480450}
4 files changed