Limit HTMLKeygenElement.keytype to only known values.

The only supported value for the <keygen/> element's keytype attribute
is the keyword for the RSA state ("rsa".) Impose the required
normalization of the IDL attribute when getting its value.

R=
BUG=321862

Review URL: https://codereview.chromium.org/123833005

git-svn-id: svn://svn.chromium.org/blink/trunk@165027 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Make the second argument to Element.setAttribute non-optional

This change broke Twitter the first time: http://crbug.com/330261

Twitter was fixed on Dec 20, 2013, so it should be safe to try again.

BUG=325922

Review URL: https://codereview.chromium.org/130193003

git-svn-id: svn://svn.chromium.org/blink/trunk@165028 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Web Animations CSS: Delay animation-start event until notification from compositor.

This aligns the timing of start events under Web Animations with the legacy
implementation. Prior to this patch Web Animations would dispatch the start
event for a compositor animation before the start time was known.

This patch moves the skipping of redundant timing updates from Player to
TimedItem allowing updateInheritedTime to be aware of when a Player gains
a start time.

This opens up opportunities to further simplify updateInheritedTime, but they
have not been attempted in this patch to ease merging this fix.

BUG=333059

Review URL: https://codereview.chromium.org/133153003

git-svn-id: svn://svn.chromium.org/blink/trunk@165030 bbb929c8-8fbe-4397-9dbb-9b2b20218538