Make sure getAttribute() / setAttribute() callers use AtomicStrings

Make sure getAttribute() / setAttribute() callers use AtomicStrings

Make sure getAttribute() / setAttribute() callers use AtomicStrings. Especially,
the DOM API should make sure to request an AtomicString if they call setAttribute()
internally so that the bindings code won't uselessly created a WTF::String from the
v8::String. That String will then implicitly be converted to an AtomicString when
calling setAttribute(), which has a cost.

This is one step towards making the AtomicString(const String&) constructor
explicit.

R=abarth, eseidel, haraken
BUG=323739
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=163293

[philipj_slow, 2013-12-05 20:10:49]

non-owner yay and lgtm for HTML{Audio,Media,Track}Element

[eseidel, 2013-12-05 21:02:39]

lgtm

[commit-bot: I haz the power, 2013-12-05 21:06:01]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ch.dumez@samsung.com/99333011/1

[commit-bot: I haz the power, 2013-12-06 01:44:36]

Change committed as 163293