Timeline: fix crash when painted RenderImage is an anonymous object

Do not expect generatingNode() to be non-null, iterate through
ancestors till we find someone with a real generating node.

BUG=387744

Review URL: https://codereview.chromium.org/358993002

git-svn-id: svn://svn.chromium.org/blink/trunk@177220 bbb929c8-8fbe-4397-9dbb-9b2b20218538
IDL: For DOMString? attribute getters, use null string to represent null

Same as we do for wrapper type attributes, where a null pointer is used
to represent a null Javascript value, use a null string (one for which
WTF::String::isNull() returns true) to represent a null Javascript value
for nullable DOMString attributes.

Review URL: https://codereview.chromium.org/338843005

git-svn-id: svn://svn.chromium.org/blink/trunk@177221 bbb929c8-8fbe-4397-9dbb-9b2b20218538