Oilpan: Fix async-script.html which was failing because we were not tracing PendingScript.

R=ager@chromium.org, mikhail.pozdnyakov@intel.com, wibling@chromium.org
BUG=

Review URL: https://codereview.chromium.org/351243002

git-svn-id: svn://svn.chromium.org/blink/trunk@177291 bbb929c8-8fbe-4397-9dbb-9b2b20218538
IDL: Drop redundant null check for string and wrapper type attributes

For nullable string and wrapper type attributes, we use a null string or
null pointer return value from the C++ getter to represent a null
Javascript value.

Rather than generating a separate null check and v8SetReturnValueNull()
call for these types, we can use v8SetReturnValueStringOrNull() for
strings, and for wrapper types, we can always use the per-type
v8SetReturnValue() overload, since they convert null pointers to null
Javascript values already.

Review URL: https://codereview.chromium.org/357413003

git-svn-id: svn://svn.chromium.org/blink/trunk@177292 bbb929c8-8fbe-4397-9dbb-9b2b20218538