Add number() static methods to AtomicString class

Add number() static methods to AtomicString class as a lot of callers need it and
previously had to use String::number() then convert the String to an AtomicString.
Calling AtomicString::number() directly when the caller needs an AtomicString is
~2% faster.

Also refactored HTML code to use the Element helper methods for Number attributes.

R=eseidel

Review URL: https://codereview.chromium.org/84713002

git-svn-id: svn://svn.chromium.org/blink/trunk@162593 bbb929c8-8fbe-4397-9dbb-9b2b20218538