Update DOMTokenList::value() to return a const reference

Update DOMTokenList::value() to return a const reference and avoid ref-counting
churn on the AtomicString.

R=adamk, esprehn

Review URL: https://codereview.chromium.org/130403008

git-svn-id: svn://svn.chromium.org/blink/trunk@165159 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove compile time flag TOUCH_ICON_LOADING and use runtime flag instead.
Update layout test case to use runtime flag.

Depends on 132173002 and 132163002

BUG=332863

Review URL: https://codereview.chromium.org/132723002

git-svn-id: svn://svn.chromium.org/blink/trunk@165160 bbb929c8-8fbe-4397-9dbb-9b2b20218538