Move hardcoded extra fields to its own JSON5 file.

third_party/WebKit/Source/core/css/ComputedStyleExtraFields.json5

+{
+// This file specifies fields in ComputedStyle that we would like to
+// generate, but are not CSS properties.
+
+  parameters: {
+    // - field_size
+    // Number of bits needed to store this field. Only used for storage_only
+    // fields. If specified, the field will be stored as bit field. Otherwise
+    // it will be stored as a normal data member.
+    field_size: {
+      valid_type: "int",
+    },
+
+    // If the field has_custom_compare_and_copy, then it does not appear in
+    // ComputedStyle::operator== and ComputedStyle::CopyNonInheritedFromCached.
+    has_custom_compare_and_copy: {
+      default: false,
+    },
+
+    // The rest is the same as CSSProperties.json5

[alancutter (OOO until 2018), 2017/04/19 07:13:32]

Full stop.

Not all parameters in CSSProperties.json5 are used here, you should list the
relevant ones explicitly.
I'd be in favour of copy pasting them here if there's not too many. WDYT?

[shend, 2017/04/19 07:38:58]

Done.
There's about ten parameters I think, and this might grow in the future. The
annoying thing is that validation information like "default" and "valid_type"
have to be copied over as well. I'm leaning towards keeping it as it is, but I
don't have a strong opinion as long as we don't have to copy the comments across
as well. A possible compromise is to list the relevant parameters in a comment.
Maybe ask for a third opinion?

[Bugs Nash, 2017/04/19 22:56:55]

If the list might grow in future then I would not include them all here, as this
list is likely to become stale. I'm ok with giving examples or listing relevant
ones

[alancutter (OOO until 2018), 2017/04/20 03:29:58]

I share your concerns for the list becoming stale. I personally think it's
better than needing the reader to trawl through generator scripts to work out
the parameters used by this file though. Not perfect though by any means.

+  },
+
+  data: [
+    {
+      name: "IsLink",
+      field_template: "monotonic_flag",
+      default_value: "false",
+      has_custom_compare_and_copy: true,
+    },
+    {
+      name: "OriginalDisplay",
+      field_template: "keyword",
+      default_value: "inline",
+      type_name: "EDisplay",
+      keywords: [
+          "inline", "block", "list-item", "inline-block", "table", "inline-table", "table-row-group", "table-header-group",
+          "table-footer-group", "table-row", "table-column-group", "table-column", "table-cell", "table-caption", "-webkit-box",
+          "-webkit-inline-box", "flex", "inline-flex", "grid", "inline-grid", "contents", "flow-root", "none",
+      ],
+      has_custom_compare_and_copy: true,
+    },
+    {
+      name: "InsideLink",
+      field_template: "keyword",
+      default_value: "not-inside-link",
+      keywords: ["not-inside-link", "inside-unvisited-link", "inside-visited-link"],
+      inherited: true,
+      has_custom_compare_and_copy: true,
+    },
+    // Style can not be shared.
+    {
+      name: "Unique",
+      field_template: "monotonic_flag",
+      default_value: "false",
+      has_custom_compare_and_copy: true,
+    },
+    // Whether this style is affected by these pseudo-classes.
+    {
+      name: "AffectedByFocus",
+      field_template: "monotonic_flag",
+      default_value: "false",
+      has_custom_compare_and_copy: true,
+    },
+    {
+      name: "AffectedByFocusWithin",
+      field_template: "monotonic_flag",
+      default_value: "false",
+      has_custom_compare_and_copy: true,
+    },
+    {
+      name: "AffectedByHover",
+      field_template: "monotonic_flag",
+      default_value: "false",
+      has_custom_compare_and_copy: true,
+    },
+    {
+      name: "AffectedByActive",
+      field_template: "monotonic_flag",
+      default_value: "false",
+      has_custom_compare_and_copy: true,
+    },
+    {
+      name: "AffectedByDrag",
+      field_template: "monotonic_flag",
+      default_value: "false",
+      has_custom_compare_and_copy: true,
+    },
+    // A non-inherited property references a variable or @apply is used
+    {
+      name: "HasVariableReferenceFromNonInheritedProperty",
+      field_template: "monotonic_flag",
+      default_value: "false",
+      has_custom_compare_and_copy: true,
+    },
+    // Explicitly inherits a non-inherited property
+    {
+      name: "HasExplicitlyInheritedProperties",
+      field_template: "monotonic_flag",
+      default_value: "false",
+      has_custom_compare_and_copy: true,
+    },
+    // These are set if we used viewport or rem units when resolving a length.
+    // TODO(shend): HasViewportUnits should be a monotonic_flag.
+    {
+      name: "HasViewportUnits",
+      field_template: "primitive",
+      default_value: "false",
+      type_name: "bool",
+      has_custom_compare_and_copy: true,
+    },
+    {
+      name: "HasRemUnits",
+      field_template: "monotonic_flag",
+      default_value: "false",
+      has_custom_compare_and_copy: true,
+    },
+    // These properties only have generated storage, and their methods are handwritten in ComputedStyle.
+    // TODO(shend): Remove these fields and delete the 'storage_only' template.
+    {
+      name: "EmptyState",
+      field_template: "storage_only",
+      field_size: 1,
+      default_value: "false",
+      type_name: "bool",
+      has_custom_compare_and_copy: true,
+    },
+    {
+      name: "StyleType",
+      field_template: "storage_only",
+      field_size: 6,
+      default_value: "0",
+      type_name: "PseudoId",
+      has_custom_compare_and_copy: true,
+    },
+    {
+      name: "PseudoBits",
+      field_template: "storage_only",
+      field_size: 8,
+      default_value: "kPseudoIdNone",
+      type_name: "PseudoId",
+      has_custom_compare_and_copy: true,
+    },
+    // True if 'underline solid' is the only text decoration on this element.
+    {
+      name: "HasSimpleUnderline",
+      field_template: "storage_only",
+      field_size: 1,
+      default_value: "false",
+      type_name: "bool",
+      inherited: true,
+      has_custom_compare_and_copy: true,
+    },
+    // TODO(shend): vertical align is actually a CSS property, but since we don't support union fields
+    // which can be either a keyword or Length, this is generated as a nonproperty for now. Remove this
+    // once we can support union fields and groups.
+    {
+      name: "VerticalAlign",
+      field_template: "storage_only",
+      field_size: 4,
+      default_value: "EVerticalAlign::kBaseline",
+      type_name: "EVerticalAlign",
+      has_custom_compare_and_copy: true,
+    },
+  ],
+}