DevTools: Support popover on timeline overview.

Source/devtools/front_end/timeline/timelinePanel.css

Index: Source/devtools/front_end/timeline/timelinePanel.css
diff --git a/Source/devtools/front_end/timeline/timelinePanel.css b/Source/devtools/front_end/timeline/timelinePanel.css
index 6ef42e8076ecce5cd8d16b0a9b51758870f0f2b8..bae8a9f5c6273b6943e0998f88922e36ac3df0b1 100644
--- a/Source/devtools/front_end/timeline/timelinePanel.css
+++ b/Source/devtools/front_end/timeline/timelinePanel.css
@@ -60,7 +60,6 @@
 }
 
 .timeline-overview-frames-mode #timeline-overview-grid,
-.timeline-overview-frames-mode .overview-grid-window-area,
 .timeline-overview-frames-mode .overview-grid-current-position {
     display: none;
 }
@@ -868,3 +867,11 @@
     max-height: 200px;
     cursor: pointer;
 }
+
+.timeline-overview-popover .frame .time {
+    display: none;
+}
+
+.timeline-overview-popover .frame .thumbnail img {
+    max-width: 200px;
+}