Replace state machine diagram SVG with PNG.

SVGs don't render in gerrit or on git links, so switch the PNG for a
better user experience.

Update reference in code and delete second redundant reference.

Change-Id: I595cc5a5403d7b6741e22bcdb47447f3aad27bc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2237533Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776819}

content/browser/frame_host/navigation_request.cc

@@ -4647,7 +4647,7 @@ void NavigationRequest::CheckStateTransition(NavigationState state) const {
   static const base::NoDestructor<StateTransitions<NavigationState>>
       transitions(StateTransitions<NavigationState>({
           // See
-          // https://chromium.googlesource.com/chromium/src/+/HEAD/docs/navigation-request-navigation-state.svg
+          // https://chromium.googlesource.com/chromium/src/+/HEAD/docs/navigation-request-navigation-state.png
           {NOT_STARTED,
            {WAITING_FOR_RENDERER_RESPONSE, WILL_START_NAVIGATION,
             WILL_START_REQUEST}},

content/browser/frame_host/navigation_request.h

@@ -110,8 +110,6 @@ class CONTENT_EXPORT NavigationRequest
  public:
   // Keeps track of the various stages of a NavigationRequest.
   // To see what state transitions are allowed, see |SetState|.
-  // See
-  // https://chromium.googlesource.com/chromium/src/+/HEAD/docs/navigation-request-navigation-state.svg
   enum NavigationState {
     // Initial state.
     NOT_STARTED = 0,