Add viewer for layout trace files.
Try to parse command line argument as file names

… when parsing as an absolute URL (without a base) fails.

Previously, arguments were parsed as URLs with the current working directory
converted to an URL and used as the base URL. This mostly worked for
relative filenames, except that `?` and `#` had a special meaning
and needed to be percent-encoded.

Fix #3340.
Add breakpoint utility function.
Give explicit file: URLs for reftests. Allow specifying a fragment id

The previous commit made #top be considered part of the file name in
../html/acid2.html#top
Store content offset in unscaled pixels

Content offset is now stored in unscaled pixels, so that it can be
applied to tile positions during compositor rendering without unscaling
it. This means that when using it in calculations of layer size (stored
in device pixels), we need to scale it. Scene scale is now stored as a
scale rather than a transformation.

Fixes #3269.
Fixes #3042.
Add unrooted_must_root lint for enums and structs containing JS<T>, as well as functions with JS<T> in their parameter list

For safe wrappers over JS<T> (eg Temporary<T>) use #[allow(unrooted_must_root)].
For all other types containing a #[must_root] value, annotate the type with #[must_root] to ensure that it is never used unrooted
Removed unused BlobMethods and CommentMethods traits, fixes #3366
Add unrooted_must_root lint for usages of JS<T> in let/for bindings
Implement background-position keyword values

Also fixes calculation of background-position percentages:

Rather than multiplying the container size by a percent and aligning the top
left of the image at the resulting width, we also need to subtract a
corresponding percent of the image size, per
http://dev.w3.org/csswg/css2/colors.html#propdef-background-position

"A value pair of '100% 100%' places the lower right corner of the image in the
lower right corner of the padding box. With a value pair of '14% 84%', the
point 14% across and 84% down the image is to be placed at the point 14%
across and 84% down the padding box."
Merge pull request #3296 from mbrubeck/background-position

Support CSS2 background-position keyword and percent values. r=SimonSapin
Unwrap pointers and miscellany for unrooted_must_root check
Merge pull request #3350 from glennw/layout-viewer

Add viewer for layout trace files.
Address review comments
Merge pull request #3365 from SimonSapin/command-line-argument-filenames

Try to parse command line argument as file names
Make Reflector #[must_root], propagate to non-HTMLElements
Use #[must_root] for HTMLElements
Merge pull request #3318 from mrobinson/scrolling

Store content offset in unscaled pixels
Update README.md
Update README.md
Merge pull request #3377 from LalehB/patch-5

Update README.md