The reason is the WMPlayer window is consisting of several sibling windows with different HWND, not child windows. So Chrome thinks it's overlapped by them.
Given those windows have same window title and from same process. Maybe we can filter them out due to these facts.

The following revision refers to this bug:
  https://webrtc.googlesource.com/src.git/+/e12e68cc4e816796ab67dce47d79dc86600d0de7

commit e12e68cc4e816796ab67dce47d79dc86600d0de7
Author: braveyao <braveyao@webrtc.org>
Date: Thu Jul 19 18:45:59 2018

[Window capture] filter out sibling windows with same title.

On Windows, some app windows (i.e. Window Media Player) seems
consisting of several sibling windows, with same window title as the
main window and from same process. Currently CroppingWindowCapturerWin
will think the selected window is overlapped by those windows and
switch to GDI capture method, which is not well supported by many Apps
on Win10 and will fail the window capture.

This cl is to extend the current null title check to include the case
that window has same title as the selected window.

Bug:  chromium:865193 
Change-Id: Id16b21596ab3b870197758679e5406138ac1a432
Reviewed-on: https://webrtc-review.googlesource.com/89501
Commit-Queue: Brave Yao <braveyao@webrtc.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#24046}
[modify] https://crrev.com/e12e68cc4e816796ab67dce47d79dc86600d0de7/modules/desktop_capture/cropping_window_capturer_win.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c0419ddbcbef45e6966ac1dbf28663bd68c8f1b0

commit c0419ddbcbef45e6966ac1dbf28663bd68c8f1b0
Author: webrtc-chromium-autoroll <webrtc-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Date: Sun Jul 22 06:08:49 2018

Roll src/third_party/webrtc 881fe53d1fae..0c3ad73948e4 (46 commits)

https://webrtc.googlesource.com/src.git/+log/881fe53d1fae..0c3ad73948e4


git log 881fe53d1fae..0c3ad73948e4 --date=short --no-merges --format='%ad %ae %s'
2018-07-21 buildbot@webrtc.org Roll chromium_revision 86c90e3417..5e7b1029e2 (576937:577041)
2018-07-20 benwright@webrtc.org Adds SSLCertificateVerifier to the Java API.
2018-07-20 mbonadei@webrtc.org Enable clang::find_bad_constructs for sdk/android (part 1/2).
2018-07-20 buildbot@webrtc.org Roll chromium_revision 7002a7d325..86c90e3417 (576827:576937)
2018-07-20 aleloi@webrtc.org Reset level estimator when analog gain changes.
2018-07-20 mbonadei@webrtc.org Enable clang::find_bad_constructs for audio_coding (part 1/2).
2018-07-20 mbonadei@webrtc.org Enable clang::find_bad_constructs for api/video/test.
2018-07-20 mbonadei@webrtc.org Enable clang::find_bad_constructs for rtc_tools (part 1/2).
2018-07-20 buildbot@webrtc.org Roll chromium_revision bc3d35fc98..7002a7d325 (576524:576827)
2018-07-20 mbonadei@webrtc.org Enable clang::find_bad_constructs for remote_bitrate_estimator.
2018-07-19 qingsi@google.com Remove MetricsObserverInterface.
2018-07-19 mbonadei@webrtc.org Enabling clang::find_bad_constructs for desktop_capture.
2018-07-19 braveyao@webrtc.org [Window capture] filter out sibling windows with same title.
2018-07-19 mbonadei@webrtc.org Reland "Removing unneeded dependency."
2018-07-19 buildbot@webrtc.org Roll chromium_revision ecaca1be48..bc3d35fc98 (576410:576524)
2018-07-19 oprypin@webrtc.org Follow "Migrate the Android Support Lib to android_deps" from Chromium
2018-07-19 yvesg@webrtc.org Expose audio codec factories in libwebrtc.
2018-07-19 aleloi@webrtc.org Fuzzer finds fixedpoint failure.
2018-07-19 mbonadei@webrtc.org Enabling clang::find_bad_constructs for libjingle_peerconnection_api.
2018-07-19 stefan@webrtc.org Rename PayloadRouter to RtpVideoSender.
2018-07-19 mbonadei@webrtc.org Enabling clang::find_bad_constructs in modules/BUILD.gn.
2018-07-19 buildbot@webrtc.org Roll chromium_revision 6374eaa6aa..ecaca1be48 (576310:576410)
2018-07-19 mbonadei@webrtc.org Clean up unneeded includes in common_video/BUILD.gn.
2018-07-19 mbonadei@webrtc.org Clean up unneeded includes in common_audio/BUILD.gn.
2018-07-19 mbonadei@webrtc.org Clean up unneeded includes in audio_device/BUILD.gn.
2018-07-19 buildbot@webrtc.org Roll chromium_revision 9e818a5aef..6374eaa6aa (576197:576310)
2018-07-19 hta@webrtc.org Add UMA metric for ICE candidate addition outcome
2018-07-19 jeroendb@webrtc.org Added explicit EOR to sctp messages and coalesce messages on the receiving side.
2018-07-18 benwright@webrtc.org Adds flag to enable AES_GCM from the Java API.
2018-07-18 buildbot@webrtc.org Roll chromium_revision 7e9fce12da..9e818a5aef (576093:576197)
2018-07-18 benwright@webrtc.org Added API to Objective-C PeerConnectionFactoryOptions to enable GCM Ciphers.
2018-07-18 buildbot@webrtc.org Roll chromium_revision 9159e523ff..7e9fce12da (575979:576093)
2018-07-18 mbonadei@webrtc.org Cleanup unneeded includes in audio_coding/BUILD.gn.
2018-07-18 sakal@webrtc.org Allow calling SoftwareVideoDecoderFactory#createDecoder(String).
2018-07-18 orphis@webrtc.org Implement changing degradation preference with setParameters()
2018-07-18 mbonadei@webrtc.org Revert "Removing unneeded dependency."
2018-07-18 stefan@webrtc.org Properly clean up RtpVideoSender.
2018-07-18 terelius@webrtc.org Limit BWE reductions before first measured throughput.
2018-07-18 mbonadei@webrtc.org Enabling clang::find_bad_constructs for modules/pacing.
2018-07-18 mbonadei@webrtc.org Removing unneeded dependency.
2018-07-18 mbonadei@webrtc.org Enabling clang::find_bad_constructs for common_video.
2018-07-18 mbonadei@webrtc.org Removing clang:find_bad_constructs from logging (part 1).
2018-07-18 mbonadei@webrtc.org Revert "Remove linux_internal_compile_lite from CQ."
2018-07-18 mbonadei@webrtc.org Adding oprypin to infra/config/OWNERS.
2018-07-18 buildbot@webrtc.org Roll chromium_revision 9ec8cfdbc9..9159e523ff (575625:575979)
2018-07-18 ilnik@webrtc.org Reland "Enable simulcast screenshare by default"


Created with:
  gclient setdep -r src/third_party/webrtc@0c3ad73948e4

The AutoRoll server is located here: https://webrtc-chromium-roll.skia.org

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.

CQ_INCLUDE_TRYBOTS=luci.chromium.try:linux_chromium_archive_rel_ng;master.tryserver.chromium.mac:mac_chromium_archive_rel_ng

BUG=chromium:None,chromium:None,chromium:None,chromium:865193,chromium:None,chromium:864883,chromium:None,chromium:None,chromium:713701,chromium:None,chromium:713701,chromium:None,chromium:None,chromium:None,chromium:None,chromium:690537
TBR=webrtc-chromium-sheriffs-robots@google.com

Change-Id: I761335ea240c6da495ddad33ad073d42bd446453
Reviewed-on: https://chromium-review.googlesource.com/1146084
Reviewed-by: webrtc-chromium-autoroll <webrtc-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Commit-Queue: webrtc-chromium-autoroll <webrtc-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#577089}
[modify] https://crrev.com/c0419ddbcbef45e6966ac1dbf28663bd68c8f1b0/DEPS

Verified with Canary  70.0.3500.0

What about the PowerPoint issue? It's so similar, but the Editor and Presentation windows have slightly different names.

There's a long standing issue here:

https://bugs.chromium.org/p/webrtc/issues/detail?id=3852

They are not similar: WMPlay case - from same process, with same title; PowerPoint case - from different process, with different title.
As comments in https://bugs.chromium.org/p/webrtc/issues/detail?id=3852, there is no way to identify the full screen window at present.