[parkable_images] Fix typo in metric name.

This Cl is a follow-up to
https://chromium-review.googlesource.com/c/chromium/src/+/2564938, which
added metrics for ParkableImages.

This CL fixed a typo ("Memory.ParkableImages.TotalSize" ->
"Memory.ParkableImage.TotalSize") which was preventing collection of
metrics.

Bug: 1154295
Change-Id: Iaa3b5cce518b761c87fc2cea719d31ab7e2a7a3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2589027Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Thiabaud Engelbrecht <thiabaud@google.com>
Cr-Commit-Position: refs/heads/master@{#837585}

third_party/blink/renderer/platform/graphics/parkable_image_manager.cc

@@ -76,7 +76,7 @@ void ParkableImageManager::Remove(DeferredImageDecoder* image) {
 void ParkableImageManager::RecordStatisticsAfter5Minutes() const {
   Statistics stats = ComputeStatistics();
 
-  base::UmaHistogramCounts100000("Memory.ParkableImages.TotalSize.5min",
+  base::UmaHistogramCounts100000("Memory.ParkableImage.TotalSize.5min",
                                  stats.total_size / 1024);  // Record in KiB.
 }