DevTools: allow resizing the sub-navigator sidebar in sources panel.

Change-Id: I2ec5bafdb4ce19336b0cc4279fdab24cfacc4b09
Reviewed-on: https://chromium-review.googlesource.com/1154431Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Commit-Queue: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579294}

third_party/blink/renderer/devtools/front_end/sources/SourcesPanel.js

@@ -81,9 +81,10 @@ Sources.SourcesPanel = class extends UI.Panel {
     if (UI.viewManager.hasViewsForLocation('run-view-sidebar')) {
       const navigatorSplitWidget = new UI.SplitWidget(false, true, 'sourcePanelNavigatorSidebarSplitViewState');
       navigatorSplitWidget.setMainWidget(tabbedPane);
-      navigatorSplitWidget.setSidebarWidget(
-          UI.viewManager.createTabbedLocation(this._revealNavigatorSidebar.bind(this), 'run-view-sidebar')
-              .tabbedPane());
+      const runViewTabbedPane =
+          UI.viewManager.createTabbedLocation(this._revealNavigatorSidebar.bind(this), 'run-view-sidebar').tabbedPane();
+      navigatorSplitWidget.setSidebarWidget(runViewTabbedPane);
+      navigatorSplitWidget.installResizer(runViewTabbedPane.headerElement());
       this.editorView.setSidebarWidget(navigatorSplitWidget);
     } else {
       this.editorView.setSidebarWidget(tabbedPane);