DevTools: Make WebGL flamechart respect the dark theme.

Change-Id: I1d2b25ee06997f3333e73e0380870d11d19843ee
Reviewed-on: https://chromium-review.googlesource.com/1179474Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584251}

third_party/blink/renderer/devtools/front_end/perf_ui/FlameChart.js

@@ -1018,8 +1018,6 @@ PerfUI.FlameChart = class extends UI.VBox {
     }
 
     gl.viewport(0, 0, this._canvasGL.width, this._canvasGL.height);
-    gl.clearColor(1.0, 1.0, 1.0, 1.0);
-    gl.clear(gl.COLOR_BUFFER_BIT);
 
     if (!this._vertexCount)
       return;