Fixed wrong return value in xr_device.idl

As following specification in url[1], return value was wrong in idl.
So I fixed the return value.

[1] https://immersive-web.github.io/webxr/#xrdevice-interface

Bug: None
Cq-Include-Trybots: luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I87adbd6e3dfe73a6a7707c282fda625b233b4d3b
Reviewed-on: https://chromium-review.googlesource.com/1155027Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Byoungkwon Ko <codeimpl@gmail.com>
Cr-Commit-Position: refs/heads/master@{#579252}

third_party/blink/renderer/modules/xr/xr_device.idl

@@ -7,6 +7,6 @@
     SecureContext,
     OriginTrialEnabled=WebXR
 ] interface XRDevice {
-  [CallWith=ScriptState, MeasureAs=XRSupportsSession] Promise supportsSession([PermissiveDictionaryConversion] optional XRSessionCreationOptions options);
-  [CallWith=ScriptState, MeasureAs=XRRequestSession] Promise requestSession([PermissiveDictionaryConversion] optional XRSessionCreationOptions options);
+  [CallWith=ScriptState, MeasureAs=XRSupportsSession] Promise<void> supportsSession([PermissiveDictionaryConversion] optional XRSessionCreationOptions options);
+  [CallWith=ScriptState, MeasureAs=XRRequestSession] Promise<XRSession> requestSession([PermissiveDictionaryConversion] optional XRSessionCreationOptions options);
 };