Clear emulated vision deficiency when disabling inspector

CDP’s Emulation#setEmulatedVisionDeficiency no longer accepts
the empty string to clear the override. This patch updates
InspectorEmulationAgent::disable accordingly.

Bug: chromium:1003700
Change-Id: I3f08aafd4b2ff0ecdf119f1d6898a8960cc3eb71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087672
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Auto-Submit: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746743}

third_party/blink/renderer/core/inspector/inspector_emulation_agent.cc

@@ -172,7 +172,7 @@ Response InspectorEmulationAgent::disable() {
   // settings to attach to the same page). TODO: support this use case.
   setEmulatedMedia(String(), {});
   if (!emulated_vision_deficiency_.Get().IsNull())
-    setEmulatedVisionDeficiency(String());
+    setEmulatedVisionDeficiency(String("none"));
   setCPUThrottlingRate(1);
   setFocusEmulationEnabled(false);
   setDefaultBackgroundColorOverride(Maybe<protocol::DOM::RGBA>());