I worry that we'll need to create a custom class to represent the CSS values on the worklet thread. Maybe we can do this for one or two types for prototyping but eventually we'll need to support cloning the original rich typed OM types.

Doing this properly I think means making a deep clone in the worklet thread's thread local storage (to support things like AtomicString). This will make supporting all of the existing types hard.

Maybe what this means is that we'll still have to run some paint worklets on the main thread for input types which we don't support serializing / cloning.

So a few half baked thoughts
 - Passing newly created CSSValues should be thread safe? I believe OffscreenCanvas did work to make sure this works. But I might be wrong.
 - If you pass HashMap<String, const CSSValue> HashMap<CSSPropertyID, const CSSStyle> it might be worth refactoring PrepopulatedStylePropertyMap to take these in the ctor, and moving other logic out(?). Maybe?
 - In the bundle converting from/to String->AtomicString seems like the best approach?
 - Images are only slightly working at the moment, and need to be fixed. There might be a better approach to the CSSImageValue than storing a CSSValue with a StyleImage.

ikilpatrick@: I don't recall how OffscreenCanvas sets the property. I think it sets the property on the HTMLCanvas that is backing the OffscreenCanvas?

OffscreenCanvas uses CSSValues internally for things like:
ctx.filter = 'blur(2px)'; // triggers CSS Parser
ctx.font = 'Arial 20px'; // same again etc.

e.g.
https://cs.chromium.org/chromium/src/third_party/blink/renderer/modules/canvas/canvas2d/base_rendering_context_2d.cc?q=base_rendering_con&sq=package:chromium&g=0&l=423

Ah, I see.

Yeah, I had a discussion with flackr@, and it seems that to make it work for us, we need to somehow "serialize" the CSSValue, pass its type (like ColorValue), and everything that is needed to rebuild that ColorValue on the worklet thread.

Does that make sense?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0a7ee900622bdbaa22f2ca4cbdb06da7ca9dcc35

commit 0a7ee900622bdbaa22f2ca4cbdb06da7ca9dcc35
Author: Xida Chen <xidachen@chromium.org>
Date: Thu Nov 29 16:00:05 2018

Make StylePropertyMapReadOnly a pure interface

This CL is implementing the Phase 1 described in this doc:
https://docs.google.com/document/d/1v-k8Ck7x_2kpBfgaVmqg2NvQRvD1PqyOt50rdO141Wc/edit#

Basically, it redefines the StylePropertyMapReadOnly to be a pure
interface. Then it renames the existing StylePropertyMapReadOnly to
StylePropertyMapReadOnlyMainThread. Then it makes all subclasses of
StylePropertyMapReadOnly to be subclasses of
StylePropertyMapReadOnlyMainThread.

This CL should not introduce any behavior change.

Bug: 895579
Change-Id: I400981d1745ab3ee80b9ba9b77c3e3406cd0c1e7
Reviewed-on: https://chromium-review.googlesource.com/c/1354118
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612216}
[modify] https://crrev.com/0a7ee900622bdbaa22f2ca4cbdb06da7ca9dcc35/third_party/blink/renderer/core/css/BUILD.gn
[modify] https://crrev.com/0a7ee900622bdbaa22f2ca4cbdb06da7ca9dcc35/third_party/blink/renderer/core/css/cssom/computed_style_property_map.h
[modify] https://crrev.com/0a7ee900622bdbaa22f2ca4cbdb06da7ca9dcc35/third_party/blink/renderer/core/css/cssom/prepopulated_computed_style_property_map.cc
[modify] https://crrev.com/0a7ee900622bdbaa22f2ca4cbdb06da7ca9dcc35/third_party/blink/renderer/core/css/cssom/prepopulated_computed_style_property_map.h
[modify] https://crrev.com/0a7ee900622bdbaa22f2ca4cbdb06da7ca9dcc35/third_party/blink/renderer/core/css/cssom/style_property_map.h
[modify] https://crrev.com/0a7ee900622bdbaa22f2ca4cbdb06da7ca9dcc35/third_party/blink/renderer/core/css/cssom/style_property_map_read_only.h
[rename] https://crrev.com/0a7ee900622bdbaa22f2ca4cbdb06da7ca9dcc35/third_party/blink/renderer/core/css/cssom/style_property_map_read_only_main_thread.cc
[add] https://crrev.com/0a7ee900622bdbaa22f2ca4cbdb06da7ca9dcc35/third_party/blink/renderer/core/css/cssom/style_property_map_read_only_main_thread.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cc7370403a6e6bbbf2178573239797a566a342a4

commit cc7370403a6e6bbbf2178573239797a566a342a4
Author: Xida Chen <xidachen@chromium.org>
Date: Tue Jan 22 18:13:28 2019

[OT-PW] Create a PaintWorkletStyleValue

This is a start of the implementation described in this doc:
https://docs.google.com/document/d/1v-k8Ck7x_2kpBfgaVmqg2NvQRvD1PqyOt50rdO141Wc/edit#heading=h.4bikug2pvlqa

In a high-level, we will be putting a
HashMap<String, std::unique_ptr<CrossThreadStyleValue>> in the
PaintWorkletStylePropertyMap, which can be safely passed cross threads,
and the PaintWorkletStyleValue can be easily converted to CSSStyleValue
which is eventually what we need on the worklet thread.

Bug: 895579
Change-Id: I986b0efc43611bc0989a9ff6d7a009fa77ba2124
Reviewed-on: https://chromium-review.googlesource.com/c/1412037
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624833}
[modify] https://crrev.com/cc7370403a6e6bbbf2178573239797a566a342a4/third_party/blink/renderer/build/scripts/core/css/properties/templates/css_property.h.tmpl
[modify] https://crrev.com/cc7370403a6e6bbbf2178573239797a566a342a4/third_party/blink/renderer/core/BUILD.gn
[modify] https://crrev.com/cc7370403a6e6bbbf2178573239797a566a342a4/third_party/blink/renderer/core/css/BUILD.gn
[add] https://crrev.com/cc7370403a6e6bbbf2178573239797a566a342a4/third_party/blink/renderer/core/css/cssom/cross_thread_style_value.h
[add] https://crrev.com/cc7370403a6e6bbbf2178573239797a566a342a4/third_party/blink/renderer/core/css/cssom/cross_thread_style_value_test.cc
[add] https://crrev.com/cc7370403a6e6bbbf2178573239797a566a342a4/third_party/blink/renderer/core/css/cssom/cross_thread_unsupported_value.cc
[add] https://crrev.com/cc7370403a6e6bbbf2178573239797a566a342a4/third_party/blink/renderer/core/css/cssom/cross_thread_unsupported_value.h
[modify] https://crrev.com/cc7370403a6e6bbbf2178573239797a566a342a4/third_party/blink/renderer/core/css/cssom/paint_worklet_style_property_map.cc
[modify] https://crrev.com/cc7370403a6e6bbbf2178573239797a566a342a4/third_party/blink/renderer/core/css/cssom/paint_worklet_style_property_map.h
[modify] https://crrev.com/cc7370403a6e6bbbf2178573239797a566a342a4/third_party/blink/renderer/core/css/cssom/paint_worklet_style_property_map_test.cc