Engage thumbnail capture code for tab hover cards.

Still to do:
* Apply the correct logic and timing for thumbnail capture (currently
  still tuned for old NTP functionality).
* Compress thumbnail data to a reasonable size.

Bug: 928954
Change-Id: I7e5c2630e8e09740b279e3c9db16014546bd833d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1526805
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642431}

chrome/browser/ui/tab_helpers.cc

@@ -73,6 +73,7 @@
 #include "chrome/browser/ui/tab_contents/core_tab_helper.h"
 #include "chrome/browser/ui/tab_dialogs.h"
 #include "chrome/browser/ui/tab_ui_helper.h"
+#include "chrome/browser/ui/thumbnails/thumbnail_tab_helper.h"
 #include "chrome/browser/ui/web_applications/web_app_metrics.h"
 #include "chrome/browser/vr/vr_tab_helper.h"
 #include "chrome/common/buildflags.h"
@@ -105,6 +106,7 @@
 #include "chrome/browser/ui/android/view_android_helper.h"
 #else
 #include "chrome/browser/banners/app_banner_manager_desktop.h"
+#include "chrome/browser/browser_features.h"
 #include "chrome/browser/plugins/plugin_observer.h"
 #include "chrome/browser/safe_browsing/safe_browsing_tab_observer.h"
 #include "chrome/browser/ui/bookmarks/bookmark_tab_helper.h"
@@ -300,6 +302,8 @@ void TabHelpers::AttachTabHelpers(WebContents* web_contents) {
   safe_browsing::SafeBrowsingTabObserver::CreateForWebContents(web_contents);
   SearchTabHelper::CreateForWebContents(web_contents);
   TabDialogs::CreateForWebContents(web_contents);
+  if (base::FeatureList::IsEnabled(features::kTabHoverCardImages))
+    ThumbnailTabHelper::CreateForWebContents(web_contents);
   web_modal::WebContentsModalDialogManager::CreateForWebContents(web_contents);
 
   if (banners::AppBannerManagerDesktop::IsEnabled())