Auto merge of #26697 - utsavoza:ugo/issue-11681/22-05-2020, r=jdm

Implement CanvasRenderingContext2d.fillText

The PR consists of broadly two main changes:
- Implementation of Canvas2dRenderingContext.font
- Basic implementation of Canvas2dRenderingContext.fillText

Although I am not fully sure about the long term goals for the canvas backend in Servo, I assumed limited scope for font and text handling (should support simple text drawing with font selection) in the current implementation as I believe a more complete implementation would eventually be brought in as a part of #22957.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #11681
- [x] There are tests for these changes

components/servo/lib.rs

@@ -879,8 +879,13 @@ fn create_constellation(
         Box::new(FontCacheWR(compositor_proxy.clone())),
     );
 
+    let (canvas_chan, ipc_canvas_chan) = CanvasPaintThread::start(
+        Box::new(CanvasWebrenderApi(compositor_proxy.clone())),
+        font_cache_thread.clone(),
+    );
+
     let initial_state = InitialConstellationState {
-        compositor_proxy: compositor_proxy.clone(),
+        compositor_proxy,
         embedder_proxy,
         debugger_chan,
         devtools_chan,
@@ -899,9 +904,6 @@ fn create_constellation(
         user_agent,
     };
 
-    let (canvas_chan, ipc_canvas_chan) =
-        CanvasPaintThread::start(Box::new(CanvasWebrenderApi(compositor_proxy)));
-
     let constellation_chan = Constellation::<
         script_layout_interface::message::Msg,
         layout_thread::LayoutThread,