First issue was: 
"ERROR ITMS-90596: "Invalid Bundle. The asset catalog at 'Payload/testflight.app/PlugIns/search_widget_extension.appex/Assets.car' can't be read. Try rebuilding the app with a non-beta version of Xcode and submit it again."

Although, it's possible this is because this submission was done from Xcode 9 (although the .app was build with Xcode 10 beta 6.)

Second issue, when submitting with Xcode 10 (so far just a warning) is:
WARNING ITMS-90737: "Invalid Document Configuration.  Document Based Apps should support either the Document Browser (UISupportsDocumentBrowser = YES) or implement Open In Place (LSSupportsOpeningDocumentsInPlace = YES/NO).  Visit https://developer.apple.com/document-based-apps/ for more information."

I'm not familiar with these plist values, anyone?

Beta build is now waiting for review from Apple. All looks good. Will close the bug when/if it's approved.

From email thread:
 6 years ago cgrigoruta@ added "Open PDF's in Bling" here:  https://chromereviews.googleplex.com/4457019 with this tracking buganizer bug: https://b.corp.google.com/issues/6437914

Assuming this is a feature we want, we need to add a flag for LSSupportsOpeningDocumentsInPlace
      When set to a value of YES, enables your app to open the original document from a file provider, rather than a copy of the document.

LSSupportsOpeningDocumentsInPlace: 
https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/LaunchServicesKeys.html#//apple_ref/doc/uid/TP40009250-SW13

Testflight build was successfully approved and released to users, but there was a warning:

Invalid Document Configuration - Document Based Apps should support either the Document Browser (UISupportsDocumentBrowser = YES) or implement Open In Place (LSSupportsOpeningDocumentsInPlace = YES/NO). Visit https://developer.apple.com/document-based-apps/ for more information.

Feel free to keep this bug open to track the warning resolution, but wanted to inform everyone that Testflight was released.

Morphing this bug to focus on LSSupportsOpeningDocumentsInPlace

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c174e35bc69b40cb38e0275ad36b8dc52b67481a

commit c174e35bc69b40cb38e0275ad36b8dc52b67481a
Author: Justin Cohen <justincohen@google.com>
Date: Wed Sep 12 14:59:06 2018

[ios] Set LSSupportsOpeningDocumentsInPlace.

Sets LSSupportsOpeningDocumentsInPlace to true.

Bug:  882035 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I64efdf2f22479da123887cc6ef21f13efdf0fed7
Reviewed-on: https://chromium-review.googlesource.com/1219947
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590680}
[modify] https://crrev.com/c174e35bc69b40cb38e0275ad36b8dc52b67481a/ios/chrome/app/resources/Info.plist

kariahda@ can we try another build tomorrow to see if this fixes the error?

lindsayw@ do we have test cases on this for validating the open pdf still works correctly from other apps?

Setting next action for tomorrow to test this.

@justincohen: We have a test case for opening a PDF in Safari and then using Open in... to open it in Chrome. I don't think we have a test case for opening a PDF in Chrome first and using Open in.. to open it in another app. Which are you asking about, the former or the latter?

The former, per https://b.corp.google.com/issues/6437914

Thanks for the clarification. The answer would be yes, we have a test case for PDF for that scenario.

The NextAction date has arrived: 2018-09-13

Here's a M71 build with this change: https://storage.cloud.google.com/chrome-signed/ios-G1N/71.0.3551.0/iphoneos12.0/ios12-sdk-canary/110/testflight.ipa?_ga=2.253453833.-1285710001.1526919341

M71 build 71.3551.0 completed processing with no warning.

This bug requires manual review: Less than 29 days to go before AppStore submit on M70
Please contact the milestone owner if you have questions.
Owners: benmason@(Android), kariahda@(iOS), geohsu@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Approved.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c5bc8b6910e121db2bac34de620f5d3c1bd626dc

commit c5bc8b6910e121db2bac34de620f5d3c1bd626dc
Author: Justin Cohen <justincohen@google.com>
Date: Thu Sep 13 22:24:33 2018

[ios] Set LSSupportsOpeningDocumentsInPlace.

Sets LSSupportsOpeningDocumentsInPlace to true.

Bug:  882035 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I64efdf2f22479da123887cc6ef21f13efdf0fed7
Reviewed-on: https://chromium-review.googlesource.com/1219947
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#590680}(cherry picked from commit c174e35bc69b40cb38e0275ad36b8dc52b67481a)
Reviewed-on: https://chromium-review.googlesource.com/1226290
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/branch-heads/3538@{#388}
Cr-Branched-From: 79f7c91a2b2a2932cd447fa6f865cb6662fa8fa6-refs/heads/master@{#587811}
[modify] https://crrev.com/c5bc8b6910e121db2bac34de620f5d3c1bd626dc/ios/chrome/app/resources/Info.plist