Stop using QualifiedName where not needed in {HTML,SVG}ElementFactory

For the common case of creating an HTML element given a localName (both
in the HTML parser and in document.createElement), avoid constructing
a QualifiedName.

The factory was already relying on the localNames for lookups, while the
the code-generation code already knew the appropriate tagName for each
factory function it generated.

We now do as much linkage between AtomicStrings and QualfiedNames as
possible during build, rather than at runtime. This saves a hash
lookup to find the QualifiedNameImpl.

R=abarth@chromium.org
BUG=316964

Review URL: https://codereview.chromium.org/68513008

git-svn-id: svn://svn.chromium.org/blink/trunk@161865 bbb929c8-8fbe-4397-9dbb-9b2b20218538