Add UMA metadata for ImageLoaderClient metrics.

Metrics were added 5 years ago in r188357, but they lack metadata.

Bug: 879446
Change-Id: I42daa58100bd8ed6c7c2d13542913e56d82f6f37
Reviewed-on: https://chromium-review.googlesource.com/1208999
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589403}

tools/metrics/histograms/histograms.xml

@@ -36065,6 +36065,37 @@ uploading your change for review.
   <summary>The time it takes to open a hyphenation dictionary file.</summary>
 </histogram>
 
+<histogram name="ImageLoader.Client.Cache.HitMiss" enum="BooleanCacheHit"
+    expires_after="2019-01-01">
+  <owner>chromeos-files-app@google.com</owner>
+  <owner>tapted@chromium.org</owner>
+  <summary>
+    For each image load request that requested caching, records whether or not
+    it was found in the client-side cache. A hit means the request was not
+    forwarded to the ImageLoader extension.
+  </summary>
+</histogram>
+
+<histogram name="ImageLoader.Client.Cache.Usage" units="%"
+    expires_after="2019-01-01">
+  <owner>chromeos-files-app@google.com</owner>
+  <owner>tapted@chromium.org</owner>
+  <summary>
+    Returns the percentage of the client-side cache that is used for loading
+    images, before they are sent to the ImageLoader extension. Expressed as a
+    percentage of ImageLoaderClient.CACHE_MEMORY_LIMIT (e.g. 20MB).
+  </summary>
+</histogram>
+
+<histogram name="ImageLoader.Client.Cached" enum="BooleanRequested"
+    expires_after="2019-01-01">
+  <owner>chromeos-files-app@google.com</owner>
+  <owner>tapted@chromium.org</owner>
+  <summary>
+    For each image load request records whether or not it requested caching.
+  </summary>
+</histogram>
+
 <histogram name="Import.ImportedHistorySize.AutoImportFromIE" units="urls">
   <owner>gcomanici@chromium.org</owner>
   <summary>