Check the return value of Platform::current()->currentThread() for media_blink_unittests.

Check the return value of Platform::current()->currentThread() for media_blink_unittests.

TestBlinkPlatformSupport class in media/blink/run_all_unittests.cc dosen't implement currentThread().
So BufferedResourceLoaderTest and BufferedDataSourceTest in media_blink_unittests are failing in the constructor of blink::TimerBase.

#0 0x0000005e802e base::debug::StackTrace::StackTrace()
#1 0x0000005e7b6f base::debug::(anonymous namespace)::StackDumpSignalHandler()
#2 0x7f9ba8f1d340 <unknown>
#3 0x000002f86d28 blink::TimerBase::TimerBase()
#4 0x0000010a5db4 blink::Timer<>::Timer()
#5 0x0000010a53aa blink::ValidationMessageClientImpl::ValidationMessageClientImpl()
#6 0x0000010a56a6 blink::ValidationMessageClientImpl::create()
#7 0x000001048049 blink::WebViewImpl::WebViewImpl()
#8 0x0000010471bb blink::WebViewImpl::create()
#9 0x000001047185 blink::WebView::create()
#10 0x00000042726b media::BufferedDataSourceTest::BufferedDataSourceTest()


BUG=506122