Fix a test in text_input_client_mac_unittest.mm.
When Chrome is linked against an OSX 10.9+ SDK, -[NSAttributedString
copy] returns a retained self, rather than a copy.
-[NSMutableAttributedString copy] returns a new object. The test
TextInputClientMacTest.GetSubstring was checking that a new object was
made, but it was passing as an input an NSAttributedString. I changed
the test to use a NSMutableAttributedString as input instead.
BUG=484203
Review URL: https://codereview.chromium.org/
1131773002
Cr-Commit-Position: refs/heads/master@{#328903}