Use built-in kMaxValue enumerator for VideoDecodeAccelerator::Recent.

Also use the two argument version of UMA_HISTOGRAM_ENUMERATION which
autodeduces the boundary value.

Bug: 742517
Change-Id: I41bb007b16869456baa16b5ab8753c1bd3e565cc
Reviewed-on: https://chromium-review.googlesource.com/989418Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547470}

components/arc/common/video_decode_accelerator.mojom

@@ -88,7 +88,6 @@ interface VideoDecodeAccelerator {
     PLATFORM_FAILURE = 4,
     INSUFFICIENT_RESOURCES = 5,
     CANCELLED = 6,
-    RESULT_MAX = 7,
   };
 
   // Initializes video decoder accelerator with specific video codec profile.

components/arc/video_accelerator/gpu_arc_video_decode_accelerator.cc

@@ -374,11 +374,8 @@ void GpuArcVideoDecodeAccelerator::Initialize(
   auto result = InitializeTask(std::move(config));
 
   // Report initialization status to UMA.
-  const int RESULT_MAX =
-      static_cast<int>(mojom::VideoDecodeAccelerator::Result::RESULT_MAX);
   UMA_HISTOGRAM_ENUMERATION(
-      "Media.GpuArcVideoDecodeAccelerator.InitializeResult",
-      static_cast<int>(result), RESULT_MAX);
+      "Media.GpuArcVideoDecodeAccelerator.InitializeResult", result);
   std::move(callback).Run(result);
 }