Media Session: move all tests but mojo/ ones to external/wpt/.

third_party/WebKit/LayoutTests/external/wpt/mediasession/README.md

Index: third_party/WebKit/LayoutTests/external/wpt/mediasession/README.md
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+# Media Session specification Tests
+
+The Media Session specification is available here: https://wicg.github.io/mediasession
+
+GitHub repository: https://github.com/WICG/mediasession
+
+File an issue: https://github.com/WICG/mediasession/issues/new
+
+## Device/system dependency
+
+The Media Session specification defines behaviors related to device or system
+features such as the actions to run when a software/hardware media key is used.
+These behaviors are not tested because they would depend on user agent specific
+implementations and device specific features.
+
+## Status of these tests
+
+These tests are not complete and only reflect the Blink tests that could be
+exported. If a reader find a behavior that could be tested and is not, they
+should feel free to file a bug.