DevTools: Restore scrolling in widgets

The behavior of `contain: layout` changed, and now it seems to prevent
scrollbars from appearing. See https://crbug.com/843320

A follow up patch will fix this for old frontends.

Bug: 851622
Change-Id: I91534d8b9f42d37766147ff31ce941916c8ed713
Reviewed-on: https://chromium-review.googlesource.com/1101404Reviewed-by: Andrey Lushnikov <lushnikov@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567885}

third_party/blink/renderer/devtools/front_end/ui/inspectorCommon.css

@@ -75,7 +75,7 @@ iframe.fill {
 .widget {
     position: relative;
     flex: auto;
-    contain: layout style;
+    contain: style;
 }
 
 .hbox {