[Mac] Fix dictionary popup in text areas.
[Mac] Fix dictionary popup in text areas.

In the case of selected text in a text area, |selection_text_| actually
stores the whole text in the text area (or at least a large range of it),
and the subset that's selected is specified via |selection_text_offset_|
and |selection_range_|. This change fixes the logic when comparing the
text to actually compare that substring instead of the complete range.

This was broken by http://crrev.com/158988.

Note: It mirrors the substring logic done by RenderWidgetHostViewMac::SelectionChanged().

BUG=154608
TEST=see bug instructions
Review URL: https://codereview.chromium.org/11269053

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@164170 0039d316-1c4b-4281-b951-d872f2087c98

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