[filesapp] Remove unused DevToolsListerBrowserTest coverage_store_

Not used (since CollectCodeCoverage creates its own coverage_store) so
we can remove this DevToolsListerBrowserTest member.

Tbr: benreich
Bug: 1113941
Change-Id: Iebe50b99bc3a0652b8c845c7311e69c5855979fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2483545Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818343}

chrome/browser/chromeos/file_manager/devtools_listener_browsertest.cc

@@ -82,7 +82,6 @@ class DevToolsListenerBrowserTest : public content::DevToolsAgentHostObserver,
   uint32_t process_id_ = 0;
   std::map<content::DevToolsAgentHost*, std::unique_ptr<DevToolsListener>>
       devtools_agent_;
-  base::FilePath coverage_store_;
 
   DISALLOW_COPY_AND_ASSIGN(DevToolsListenerBrowserTest);
 };