Extend SplitStringIntoKeyValuePairs unit tests.

Extends the unit tests about corner cases to ensure that changes to the implementation don't break these.
Test cases of the internal helper function SplitStringIntoKeyValues are merged into tests of the public function SplitStringIntoKeyValuesPairs. This removes redundancy in the tests and improves coverage of SplitStringIntoKeyValuesPairs.

The helper function is removed from the public header.

BUG=NONE

Review URL: https://codereview.chromium.org/184233009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255758 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255758