Rename fake_openvr_impl_api variables

Renames the variables in fake_openvr_impl_api.cc to be consistent with
Chromium style guidelines instead of using the original names from
openvr.h.

Also drive-by fixes a timeout that was flakily getting hit.

Bug: 892717
Change-Id: Id6f25826df26212be5e0fae659cc4715c459f093
Reviewed-on: https://chromium-review.googlesource.com/c/1312195Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604677}

chrome/browser/vr/webxr_vr_pixel_browser_test.cc

@@ -64,7 +64,7 @@ void TestPresentationPixelsImpl(WebXrVrBrowserTestBase* t,
 
   // Wait for JavaScript to submit at least one frame.
   EXPECT_TRUE(
-      t->PollJavaScriptBoolean("hasPresentedFrame", t->kPollTimeoutShort))
+      t->PollJavaScriptBoolean("hasPresentedFrame", t->kPollTimeoutMedium))
       << "No frame submitted";
 
   // Tell JavaScript that it is done with the test.