Script task gets resize message for unknown pipeline on yahoo.com

[jdm]

With #3326, I see task 'ScriptTask' failed at 'resize sent to nonexistent pipeline', /Users/jack/src/rust/src/libcore/option.rs:243

Breakpoint 1, 0x000000010130dae0 in rust_fail ()
(gdb) bt
#0  0x000000010130dae0 in rust_fail ()
#1  0x000000010130e378 in unwind::begin_unwind_inner::h1a639982204c4870H5d ()
#2  0x000000010130deab in unwind::begin_unwind_fmt::h6fe5edd7485e5da1c3d ()
#3  0x000000010130dd02 in rust_begin_unwind ()
#4  0x000000010133002c in failure::begin_unwind::h17207e24a21fb26cc0j ()
#5  0x0000000100929b0d in option::Option$LT$T$GT$::expect::h17854245097308143607 ()
#6  0x000000010091495f in script_task::ScriptTask::handle_msgs::he819ce13b5009061gbq ()
#7  0x0000000100913da2 in script_task::ScriptTask::start::ha80c997f24e052a56aq ()
#8  0x00000001000f01ad in script_task::ScriptTask.ScriptTaskFactory::create::closure.10152 ()
#9  0x00000001000ebf66 in task::TaskBuilder$LT$S$GT$::try_future::closure.10063 ()
#10 0x0000000100d0523c in task::bootstrap_green_task::closure.6732 ()
#11 0x000000010130ef7c in rust_try ()
#12 0x000000010130bb8b in unwind::try::h8330e3ff130bb07c6Td ()
#13 0x000000010130b96b in task::Task::run::h6f43cbc2f16c1f21e2c ()
#14 0x0000000100d050b5 in task::bootstrap_green_task::h5080d69635400517Pdc ()
#15 0x0000000000000000 in ?? ()