Fix test case table in HistoryURLProviderTest for C++11

C++11 narrowing rules do not accept converting signed literals (-1) to
unsigned types (size_t), so the table for SuggestExactInput test breaks
compilation when using G++ 4.7.2.
This patch uses a localized version of std::string::npos instead of the
literal -1.

TEST=unit_tests --gtest_filter="*SuggestExactInput"

Review URL:

git-svn-id: svn:// 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed