This seems to have been caused by the upgrade to VrCore 1.15 (https://chromium-review.googlesource.com/c/chromium/src/+/1105515), as I can no longer reproduce when switching back to 1.14. I'll file a bug against VrCore.

Filed b/110438384

Duplicating some information here from the VrCore bug for clarity:

The problem seems to be that launchInVr is flakily not actually putting us into VR before launching the given intent, causing this check https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/vr/VrMainActivity.java?q=vrmainactivity&sq=package:chromium&dr=CSs&l=49 to fail sometimes because isInVrSession returns false.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ffff67f6f055730e24c384f92215f2326fecb3ce

commit ffff67f6f055730e24c384f92215f2326fecb3ce
Author: bsheedy <bsheedy@chromium.org>
Date: Mon Jun 25 17:34:18 2018

Add VR workarounds for early entry

Adds two workarounds for Chrome being resumed or started too early when
entering VR. These can be removed if/when the platform-side issue that
causes these issues is fixed in VrCore.

Bug:  854327 , 762724
Change-Id: If9645c58e9d208a734582992aad5c85d378f5f40
Reviewed-on: https://chromium-review.googlesource.com/1112761
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570084}
[modify] https://crrev.com/ffff67f6f055730e24c384f92215f2326fecb3ce/chrome/android/java/src/org/chromium/chrome/browser/vr/VrMainActivity.java
[modify] https://crrev.com/ffff67f6f055730e24c384f92215f2326fecb3ce/chrome/android/java/src/org/chromium/chrome/browser/vr_shell/VrIntentUtils.java
[modify] https://crrev.com/ffff67f6f055730e24c384f92215f2326fecb3ce/chrome/android/javatests/src/org/chromium/chrome/browser/vr_shell/util/TransitionUtils.java

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1b3df93c5adbf3768d6e3896a0431314708c973b

commit 1b3df93c5adbf3768d6e3896a0431314708c973b
Author: Theresa <twellington@chromium.org>
Date: Mon Jun 25 23:21:34 2018

Revert "Add VR workarounds for early entry"

This reverts commit ffff67f6f055730e24c384f92215f2326fecb3ce.

Reason for revert: Suspected to be causing test failures on Nougat bot

Bug:  854327 , 762724, 856373

Original change's description:
> Add VR workarounds for early entry
> 
> Adds two workarounds for Chrome being resumed or started too early when
> entering VR. These can be removed if/when the platform-side issue that
> causes these issues is fixed in VrCore.
> 
> Bug:  854327 , 762724
> Change-Id: If9645c58e9d208a734582992aad5c85d378f5f40
> Reviewed-on: https://chromium-review.googlesource.com/1112761
> Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
> Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#570084}

TBR=mthiesse@chromium.org,bsheedy@chromium.org

Change-Id: I714637798cc2944a93b33240360fb27fc6dc0ee4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug:  854327 , 762724
Reviewed-on: https://chromium-review.googlesource.com/1113846
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570231}
[modify] https://crrev.com/1b3df93c5adbf3768d6e3896a0431314708c973b/chrome/android/java/src/org/chromium/chrome/browser/vr/VrMainActivity.java
[modify] https://crrev.com/1b3df93c5adbf3768d6e3896a0431314708c973b/chrome/android/java/src/org/chromium/chrome/browser/vr_shell/VrIntentUtils.java
[modify] https://crrev.com/1b3df93c5adbf3768d6e3896a0431314708c973b/chrome/android/javatests/src/org/chromium/chrome/browser/vr_shell/util/TransitionUtils.java

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/82ba5f3a55697933e46bc391022b0f425db0152f

commit 82ba5f3a55697933e46bc391022b0f425db0152f
Author: bsheedy <bsheedy@chromium.org>
Date: Tue Jun 26 16:40:28 2018

Reland "Add VR workarounds for early entry"

This is a reland of ffff67f6f055730e24c384f92215f2326fecb3ce

Original change's description:
> Add VR workarounds for early entry
>
> Adds two workarounds for Chrome being resumed or started too early when
> entering VR. These can be removed if/when the platform-side issue that
> causes these issues is fixed in VrCore.
>
> Bug:  854327 , 762724
> Change-Id: If9645c58e9d208a734582992aad5c85d378f5f40
> Reviewed-on: https://chromium-review.googlesource.com/1112761
> Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
> Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#570084}

Bug:  854327 , 762724, 856373
Change-Id: Ifc7bd0d0a460fe1defa05a13ce20593b2662babd
Reviewed-on: https://chromium-review.googlesource.com/1114046
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570426}
[modify] https://crrev.com/82ba5f3a55697933e46bc391022b0f425db0152f/chrome/android/java/src/org/chromium/chrome/browser/vr/VrMainActivity.java
[modify] https://crrev.com/82ba5f3a55697933e46bc391022b0f425db0152f/chrome/android/java/src/org/chromium/chrome/browser/vr_shell/VrIntentUtils.java
[modify] https://crrev.com/82ba5f3a55697933e46bc391022b0f425db0152f/chrome/android/javatests/src/org/chromium/chrome/browser/vr_shell/util/TransitionUtils.java

Workaround seems to have fixed the flakiness. Leaving this open as an ExternalDependency until it's fixed at the platform level.

DLAs have been removed.