[filesapp] Create DevToolsListenerBrowserTest tmp_dir_ when used

Only CollectCodeCoverage() needs tmp_dir_ so create it there (when the
browser test is collecting code coverage).

Tbr: benreich
Bug: 1113941
Change-Id: Ib92b271ec76f8c6f42f4f704467bc00cbf3b688a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2483984Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818342}

chrome/browser/chromeos/file_manager/devtools_listener_browsertest.cc

@@ -33,9 +33,6 @@ class DevToolsListenerBrowserTest : public content::DevToolsAgentHostObserver,
   void SetUpOnMainThread() override {
     process_id_ = base::GetUniqueIdForProcess().GetUnsafeValue();
     content::DevToolsAgentHost::AddObserver(this);
-
-    base::ScopedAllowBlockingForTesting allow_blocking;
-    CHECK(tmp_dir_.CreateUniqueTempDir());
   }
 
   bool ShouldForceDevToolsAgentHostCreation() override { return true; }
@@ -65,6 +62,7 @@ class DevToolsListenerBrowserTest : public content::DevToolsAgentHostObserver,
   void CollectCodeCoverage() {
     base::ScopedAllowBlockingForTesting allow_blocking;
 
+    CHECK(tmp_dir_.CreateUniqueTempDir());
     base::FilePath coverage_store =
         tmp_dir_.GetPath().AppendASCII("devtools_listener_browser_test");
     CHECK(base::CreateDirectory(coverage_store));