From the internal bug:

I didn't find matching Chrome crash reports, but from the initial investigation:
- All the crash reports I looked at where from Android 5.0.x, API Level 21, no matter the phone model.
- There is very little code executed synchronously in warmup(), narrowing down the possibilities

Basically, what warmup() does synchronously is:
1. Check whether the first run experience should be ran. This involves a special check for the intent creator for GSA
2. Check whether the calling app is foreground
3. Post a task to the main thread

(3) cannot be the issue, since posting a task to the main looper is always OK, and the code executing in the task is asynchronous.
(2) has two different code paths, depending on whether Chrome is running on a system with API level <= 21 or not. However this code hasn't changed since M45.
(1) is somewhat new, at least part of it.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f974fe4e5a32803b83592a014a82db683cb485f7

commit f974fe4e5a32803b83592a014a82db683cb485f7
Author: lizeb <lizeb@chromium.org>
Date: Tue Oct 11 11:56:33 2016

customtabs: Add paranoia to isCallerForegroundOrSelf() due to NPEs in the wild.

We have a number of NPEs reported from devices in the
wild. Unfortunately, we only know that they happen on L pre-MR1, and
from warmup(). This commits adds null checks to prevent these from
crashing the client (reported by GSA).

BUG= 654705 

Review-Url: https://codereview.chromium.org/2404183004
Cr-Commit-Position: refs/heads/master@{#424406}

[modify] https://crrev.com/f974fe4e5a32803b83592a014a82db683cb485f7/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabsConnection.java

Requesting a merge to M55. Rationale: crash visible to users.

Your change meets the bar and is auto-approved for M55 (branch: 2883)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1d5357e02cda8ed82b7788b76e036937a329460f

commit 1d5357e02cda8ed82b7788b76e036937a329460f
Author: Benoit Lize <lizeb@chromium.org>
Date: Wed Oct 12 14:05:15 2016

customtabs: Add paranoia to isCallerForegroundOrSelf() due to NPEs in the wild.

We have a number of NPEs reported from devices in the
wild. Unfortunately, we only know that they happen on L pre-MR1, and
from warmup(). This commits adds null checks to prevent these from
crashing the client (reported by GSA).

BUG= 654705 

Review-Url: https://codereview.chromium.org/2404183004
Cr-Commit-Position: refs/heads/master@{#424406}
(cherry picked from commit f974fe4e5a32803b83592a014a82db683cb485f7)

Review URL: https://codereview.chromium.org/2415573002 .

Cr-Commit-Position: refs/branch-heads/2883@{#59}
Cr-Branched-From: 614d31daee2f61b0180df403a8ad43f20b9f6dd7-refs/heads/master@{#423768}

[modify] https://crrev.com/1d5357e02cda8ed82b7788b76e036937a329460f/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabsConnection.java

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1d5357e02cda8ed82b7788b76e036937a329460f

commit 1d5357e02cda8ed82b7788b76e036937a329460f
Author: Benoit Lize <lizeb@chromium.org>
Date: Wed Oct 12 14:05:15 2016

customtabs: Add paranoia to isCallerForegroundOrSelf() due to NPEs in the wild.

We have a number of NPEs reported from devices in the
wild. Unfortunately, we only know that they happen on L pre-MR1, and
from warmup(). This commits adds null checks to prevent these from
crashing the client (reported by GSA).

BUG= 654705 

Review-Url: https://codereview.chromium.org/2404183004
Cr-Commit-Position: refs/heads/master@{#424406}
(cherry picked from commit f974fe4e5a32803b83592a014a82db683cb485f7)

Review URL: https://codereview.chromium.org/2415573002 .

Cr-Commit-Position: refs/branch-heads/2883@{#59}
Cr-Branched-From: 614d31daee2f61b0180df403a8ad43f20b9f6dd7-refs/heads/master@{#423768}

[modify] https://crrev.com/1d5357e02cda8ed82b7788b76e036937a329460f/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabsConnection.java

[Automated comment] removing mislabelled merge-merged-2840

[Automated comment] removing mislabelled merge-merged-2840