Merge CustomElement::isCustomTagName and isValidTypeName...

...and name it simply isValidName, which nicely reflects
Document::isValidName.

In theory custom tag names and type extension names could be separate
sets. However that's not how the spec or implementation are structured
today, so having a distinction in the Custom Elements interface is
specious, untested, etc.

Refactoring; no change in behavior.

BUG=234509

Review URL: https://chromiumcodereview.appspot.com/23281002

git-svn-id: svn://svn.chromium.org/blink/trunk@156198 bbb929c8-8fbe-4397-9dbb-9b2b20218538