LibWeb: Implement CanvasRenderingContext2D.isContextLost()

Note that we don't implement the "context lost steps" yet, so this will
always return the initial value (false).

Userland/Libraries/LibWeb/HTML/CanvasRenderingContext2D.h

@@ -72,6 +72,7 @@ public:
     void save();
     void restore();
     void reset();
+    bool is_context_lost();
 
     void reset_to_default_state();
 
@@ -97,6 +98,9 @@ private:
     DrawingState m_drawing_state;
     Vector<DrawingState> m_drawing_state_stack;
 
+    // https://html.spec.whatwg.org/multipage/canvas.html#concept-canvas-context-lost
+    bool m_context_lost { false };
+
     Gfx::Path m_path;
 };