Fixed StyleSheetCollection::compareStyleSheets to not return Additive.

In the new test case, we are not adding a new stylesheet, but this code
would return Additive anyways. It has an off-by-one, where it needs
to check newIndex == newStyleSheetCount at the end of the for-loop.

I couldn't come up with a testcase where this shows a bug because
we always would end up coming through again an returning Reconstruct, but
the test included with this patch hits the buggy codepath. This
becomes a problem for RuleSets living of StyleSheetContents because
we won't process them correctly in the Additive case.

Also, fixed naming to consistently capitalize the S in Sheet.

The style-scoped test result is now wrong, but it's just exposing
an existing bug. If you load this page in content_shell and then
open the inspector, you get the same broken rendering. The old
code just happened to be passing.

I don't fully understand the fast/forms change, but since forcing
a layout fixes it, I'm assuming it's just a timing difference.

Review URL: https://codereview.chromium.org/66993010

git-svn-id: svn://svn.chromium.org/blink/trunk@162393 bbb929c8-8fbe-4397-9dbb-9b2b20218538