Commands mountFakeUsb and mountFakeMtp: assert their Mount() works

These commands create then mount fake USB and MTP volumes for testing.
Add ASSERTs that Mount() calls on these fake volumes succeeds in test.

No change in behavior, no new tests.

Bug: 833834
Change-Id: I49c9a21b629cdb6285982d622e1020b6c16dee73
Reviewed-on: https://chromium-review.googlesource.com/1042005Reviewed-by: Stuart Langley <slangley@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555900}

chrome/browser/chromeos/file_manager/file_manager_browsertest_base.cc

@@ -738,7 +738,7 @@ void FileManagerBrowserTestBase::OnCommand(const std::string& name,
     usb_volume_.reset(new FakeTestVolume("fake-usb",
                                          VOLUME_TYPE_REMOVABLE_DISK_PARTITION,
                                          chromeos::DEVICE_TYPE_USB));
-    usb_volume_->Mount(profile());
+    ASSERT_TRUE(usb_volume_->Mount(profile()));
     return;
   }
 
@@ -747,7 +747,7 @@ void FileManagerBrowserTestBase::OnCommand(const std::string& name,
                                          chromeos::DEVICE_TYPE_UNKNOWN));
     ASSERT_TRUE(mtp_volume_->PrepareTestEntries(profile()));
 
-    mtp_volume_->Mount(profile());
+    ASSERT_TRUE(mtp_volume_->Mount(profile()));
     return;
   }