Add feature flag for pending download improvement.

Bug: 796324
Change-Id: I915258edeb621eab5034b6404a8c06621dde1432
Reviewed-on: https://chromium-review.googlesource.com/835284
Reviewed-by: Dmitry Titov <dimich@chromium.org>
Reviewed-by: Yafei Duan <romax@chromium.org>
Commit-Queue: Candice Sy <cmsy@google.com>
Cr-Original-Commit-Position: refs/heads/master@{#525530}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 229768e0bd8d4e2f084509abafdbaf91c656fd11

metrics/histograms/enums.xml

@@ -25273,6 +25273,7 @@ from previous Chrome versions.
   <int value="-474322576" label="disable-quick-unlock-pin"/>
   <int value="-472013317" label="WebRTC-H264WithOpenH264FFmpeg:disabled"/>
   <int value="-471405972" label="ViewsProfileChooser:disabled"/>
+  <int value="-470948890" label="OfflinePagesDescriptivePendingStatus:enabled"/>
   <int value="-462205750" label="enable-service-worker-sync"/>
   <int value="-461292699" label="ContentSuggestionsCategoryOrder:enabled"/>
   <int value="-460313418" label="ProgressBarThrottle:enabled"/>
@@ -25417,6 +25418,7 @@ from previous Chrome versions.
   <int value="-69427025" label="OfflinePagesPrefetchingUI:enabled"/>
   <int value="-68877684" label="BackgroundVideoTrackOptimization:enabled"/>
   <int value="-68225452" label="enable-translate-new-ux"/>
+  <int value="-67297229" label="OfflinePagesDescriptivePendingStatus:disabled"/>
   <int value="-59401847" label="ContentSuggestionsLargeThumbnail:disabled"/>
   <int value="-58242474" label="ash-disable-swipe-to-close-in-overview-mode"/>
   <int value="-57986995" label="DisablePostScriptPrinting:enabled"/>