Dumping named grid line should use the author's order

Before this change, we were using a semi-random order (based
on the hashes' ordering inside the HashMap) to return the
named grid lines in getComputedStyle. This made a test
fragile and ultimately break under ASAN (because we
don't have the same default size for HashMaps).

The unspecified order also introduces a potential web
compatibility risk. Tab Atkins was notified to make sure
that it is corrected.

BUG=270300

Review URL: https://chromiumcodereview.appspot.com/22929020

git-svn-id: svn://svn.chromium.org/blink/trunk@156474 bbb929c8-8fbe-4397-9dbb-9b2b20218538