[api-minor] Improve pdf reading in high contrast mode

- Use Canvas & CanvasText color when they don't have their default value
  as background and foreground colors.
- The colors used to draw (stroke/fill) in a pdf are replaced by the bg/fg
  ones according to their luminance.

extensions/chromium/preferences_schema.json

@@ -208,6 +208,16 @@
         2
       ],
       "default": -1
+    },
+    "pageBackgroundColor": {
+      "description": "The color is a string as defined in CSS. Its goal is to help improve readability in high contrast mode",
+      "type": "string",
+      "default": "Canvas"
+    },
+    "pageForegroundColor": {
+      "description": "The color is a string as defined in CSS. Its goal is to help improve readability in high contrast mode",
+      "type": "string",
+      "default": "CanvasText"
     }
   }
 }