script: Squash duplicate animation tick events in the queue.
This prevents them from piling up endlessly if the JS
`requestAnimationFrame()` callback takes a long time to complete.
r? @jdm
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/6841)
<!-- Reviewable:end -->
This is used for two memory reporting improvements.
- It's used to distinguish "explicit" memory reports from others. This
mirrors the same categorization that is used in Firefox, and gives a single
tree that's the best place to look. It replaces the "pages" tree which
was always intended to be a temporary stand-in for "explicit".
- It's used to computed "heap-unclassified" values for both the jemalloc
and system heaps, both of which are placed into the "explicit" tree.
Example output:
```
| 114.99 MiB -- explicit
| 52.34 MiB -- jemalloc-heap-unclassified
| 46.14 MiB -- system-heap-unclassified
| 14.95 MiB -- url(file:///home/njn/moz/servo2/../servo-static-suite/wikipe
dia/Guardians%20of%20the%20Galaxy%20(film)%20-%20Wikipedia,%20the%20free%20encyc
lopedia.html)
| 7.32 MiB -- js
| 3.07 MiB -- malloc-heap
| 3.00 MiB -- gc-heap
| 2.49 MiB -- used
| 0.34 MiB -- decommitted
| 0.09 MiB -- unused
| 0.09 MiB -- admin
| 1.25 MiB -- non-heap
| 1.36 MiB -- layout-worker-3-local-context
| 1.34 MiB -- layout-worker-0-local-context
| 1.24 MiB -- layout-worker-1-local-context
| 1.24 MiB -- layout-worker-4-local-context
| 1.16 MiB -- layout-worker-2-local-context
| 0.89 MiB -- layout-worker-5-local-context
| 0.38 MiB -- layout-task
| 0.31 MiB -- display-list
| 0.07 MiB -- local-context
| 1.56 MiB -- compositor-task
| 0.78 MiB -- surface-map
| 0.78 MiB -- layer-tree
```
The heap-unclassified values dominate the "explicit" tree because reporter
coverage is still quite poor.
Fix a few issues
- Use SmallVec<[T; N]>
- Make find_iframe a free function
- Make ProgressEvent use enums for bubbles and cancelable
- Change README, as `rust-snapshot-hash` is just a text file
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/6682)
<!-- Reviewable:end -->
process parts.
This will make it easier to adapt to IPC.
The trickiest part here was to make script tasks spawn new layout tasks
directly instead of having the pipeline do it for them. The latter
approach will not work in multiprocess mode, because layout and script
must run in the same address space and the pipeline cannot inject tasks
into another process.
an IPC channel instead.
Because this used a boxed trait object to invoke messages across a
process boundary, and boxed trait objects are not supported across IPC,
we spawn a helper thread inside the compositor to perform the marshaling
for us.
image fragments.
This also changes the way the placeholder is handled in the image cache
task to decode it up front instead of each time an image fails to load,
both because it was more convenient to implement that way and because
it saves CPU cycles to do so.
This matches the behavior of Gecko and WebKit. It improves the look of
our cached copy of Wikipedia.
