DevTools: allow zoom in Sources when not paused

When debugger is paused, holding Ctrl/Cmd highlights
Continue-to-location markers, and DevTools prevents scrolling from
adjusting zoom.

This CL checks whether we are paused somewhere, and allows zooming
via Ctrl-Wheel when not paused.

Bug: 878400
Change-Id: Ibe664f5241914b89fff9c5d7ae90658f2d803918
Reviewed-on: https://chromium-review.googlesource.com/1205537Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Commit-Queue: Erik Luo <luoe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589041}

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

@@ -75,7 +75,7 @@ Sources.DebuggerPlugin = class extends Sources.UISourceCodeFrame.Plugin {
     this._boundBlur = this._onBlur.bind(this);
     this._textEditor.element.addEventListener('focusout', this._boundBlur, false);
     this._boundWheel = event => {
-      if (UI.KeyboardShortcut.eventHasCtrlOrMeta(event))
+      if (this._executionLocation && UI.KeyboardShortcut.eventHasCtrlOrMeta(event))
         event.preventDefault();
     };
     this._textEditor.element.addEventListener('wheel', this._boundWheel, true);