Rename LoadIframeUrlMsg to CreateIFrameAndLoadUrlMsg

This better reflects what the message does.
Small fix to variable name

This is a more accurate name for the script pipeline.
Add some factory methods for frame tree types
Send incremental frame tree updates to the compositor

This allows the compositor to add frames after the call to SetIds,
where the initial frame tree is created. There are still some issues
preventing proper late frame creation, but this prevents crashes when
it happens.

Fixes #3738.
auto merge of #3951 : mrobinson/servo/iframes, r=jdm

This is the first step to allowing incremental iframe creation and destruction. This should eliminate task failures when an iframe is added to the frame tree lazily via script.
Sending exit message to devtools to exit.

Accepted and closed incoming streams

Added header to for constant use.

Removed extra spaces.

Corrected spacing to conform to coding standards.

Corrected spacing to conform to coding standards.

Corrected spacing to conform with coding standards.

Moving new import next to old import for devtools_traits as specified in comment.

Removed method definition to include as inline code and changed to iter_mut.

Using loops to exit devtools on server exit message and disconnected message to simply break as suggested in the comment.

Removing TODO comment for completed functionality.

Moved the operation for exit of devtools on servo closing outside the match loop.

Removing trailing ';' and adding new line before connection closing loop.