[Chromecast] Fix timestamp checks in CMA unit tests

CMA backends might choose to implement GetCurrentPts in such a way that
it reports the current position to be kStartPts immediately after
Start(kStartPts) is called, rather then after playback actually starts,
and that should be ok.

Bug: none
Test: cast_media_unittests
Change-Id: I4e232504c0a664fb0e715eed96982c2a6d62b2e2
Reviewed-on: https://chromium-review.googlesource.com/775873Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Sergey Volk <servolk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517493}

chromecast/media/cma/backend/audio_video_pipeline_device_unittest.cc

@@ -795,12 +795,14 @@ void AudioVideoPipelineDeviceTest::MonitorLoop() {
     if (audio_feeder_ &&
         audio_feeder_->last_pushed_pts() !=
             std::numeric_limits<int64_t>::min()) {
-      EXPECT_LE(pts, audio_feeder_->last_pushed_pts() + 100 * 1000);
+      EXPECT_LE(pts, std::max(kStartPts,
+                              audio_feeder_->last_pushed_pts() + 100 * 1000));
     }
     if (video_feeder_ &&
         video_feeder_->last_pushed_pts() !=
             std::numeric_limits<int64_t>::min()) {
-      EXPECT_LE(pts, video_feeder_->last_pushed_pts() + 100 * 1000);
+      EXPECT_LE(pts, std::max(kStartPts,
+                              video_feeder_->last_pushed_pts() + 100 * 1000));
     }
     // PTS is allowed to move backwards once to allow for updates when the first
     // buffers are pushed.