prerender: Remove unneeded explicit values for PrerenderMode enum.

As far as I can see, there's no need to assign these specific values,
so remove them.

Change-Id: I10c79593c0a115f9f5cf672ea085d2390f5c6083
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2395316Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805224}

components/prerender/common/prerender_types.mojom

@@ -6,8 +6,8 @@ module prerender.mojom;
 
 enum PrerenderMode {
   // Neither prefetch nor prerender.
-  kNoPrerender = 0,
+  kNoPrerender,
 
   // Prefetch some network resources to warm up the cache.
-  kPrefetchOnly = 2,
+  kPrefetchOnly,
 };

components/prerender/renderer/prerender_helper.h

@@ -60,7 +60,7 @@ class PrerenderHelper
   void OnThrottleDestroyed();
   void SendPrefetchFinished();
 
-  prerender::mojom::PrerenderMode prerender_mode_;
+  const prerender::mojom::PrerenderMode prerender_mode_;
   std::string histogram_prefix_;
 
   // Pending requests for this frame..