[fuchsia] Minor fixes in TestComponentContextForProcess

This patch:

 - fixes a comment which I think meant kCloneAll where it says kEmpty
 - makes the single arg constructor explicit

Bug: None
Change-Id: Id0fb554a3caf06cb2d655fb06a8307cfedc2595a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547302Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Przemek Pietrzkiewicz <ppi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829175}

base/fuchsia/test_component_context_for_process.h

@@ -53,11 +53,11 @@ class FilteredServiceDirectory;
 //   test_context.AddServices({fuchsia::memorypressure::Provider::Name_, ...});
 //   // ... Execute tests which use fuchsia.memorypressure.Provider ...
 //
-// Alternatively InitialState::kEmpty can be passed to the constructor to expose
-// all services listed in /svc, e.g.:
+// Alternatively InitialState::kCloneAll can be passed to the constructor to
+// expose all services listed in /svc, e.g.:
 //
 //   TestComponentContextForProcess test_context(
-//       TestComponentContextForProcess::InitialState::kEmpty);
+//       TestComponentContextForProcess::InitialState::kCloneAll);
 //
 // Fake/mock implementations can be exposed via additional_services():
 //
@@ -79,7 +79,7 @@ class BASE_EXPORT TestComponentContextForProcess {
     kCloneAll,
   };
 
-  TestComponentContextForProcess(
+  explicit TestComponentContextForProcess(
       InitialState initial_state = InitialState::kEmpty);
   ~TestComponentContextForProcess();