Added some discussion about the wording of this dialogue to  https://crbug.com/587306#c23 . I will reuse the same dialog as when you launch an intent from an incognito window, with the wording changed for clarity. The proposed wording is:

"Leave incognito mode?

You are about to send data to an external application.

[Cancel] [Continue]"

See attached screenshot.

CL (missing tests): https://codereview.chromium.org/2354833002

Deleted: incognito-webshare.png 143 KB

	incognito-webshare.png 143 KB View Download

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7db85418178d9d4f01a7901da7450e40e08414ec

commit 7db85418178d9d4f01a7901da7450e40e08414ec
Author: mgiuca <mgiuca@chromium.org>
Date: Fri Sep 23 03:19:28 2016

Web Share: Show a warning prompt when share is used from incognito.

This is the same dialog as shown when external links are opened from
incognito, informing the user that they are leaving incognito mode and
potentially sharing data with an external application.

BUG= 645007 

Review-Url: https://codereview.chromium.org/2354833002
Cr-Commit-Position: refs/heads/master@{#420564}

[modify] https://crrev.com/7db85418178d9d4f01a7901da7450e40e08414ec/chrome/android/java/src/org/chromium/chrome/browser/externalnav/ExternalNavigationDelegateImpl.java
[modify] https://crrev.com/7db85418178d9d4f01a7901da7450e40e08414ec/chrome/android/java/src/org/chromium/chrome/browser/webshare/ShareServiceImpl.java

Works as per expected behavior. Confirmation dialog to when shring through incognito. 
Issue is verified on Latest M55 Build on trunk.

This went through UX review and it was decided that we don't need it (the intent picker shown by the Android system is sufficient notice to the user, with the Share menu picker being used as precedent).

Reverting this change.

Raised by tedchoc@ on code review: yes, we will still see a chooser even if there is only one possible target.

I simulated this by turning on CATEGORY_BROWSABLE in the source code; then I only have one app (Intent Intercept) that can handle the intent. You can see this one-app picker here.

Deleted: share-one-app.png 181 KB

	share-one-app.png 181 KB View Download

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e27d189dc63ecb32c2645828557389de11e7a466

commit e27d189dc63ecb32c2645828557389de11e7a466
Author: mgiuca <mgiuca@chromium.org>
Date: Thu Oct 06 23:41:46 2016

Web Share: Disable warning when web share is used from incognito.

This was deemed unnecessary in UX review. When navigator.share is used
from incognito, you still see the intent picker which is considered
sufficient warning for users.

The bulk of the code to support this (https://crrev.com/420564) remains
and will be removed in a future CL.

BUG= 645007 

Review-Url: https://codereview.chromium.org/2389393004
Cr-Commit-Position: refs/heads/master@{#423740}

[modify] https://crrev.com/e27d189dc63ecb32c2645828557389de11e7a466/chrome/android/java/src/org/chromium/chrome/browser/webshare/ShareServiceImpl.java

Now that this has been landed and reverted, marking as WontFix (the original issue).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/850d38c2fcc1a887aa0f38319eb1b8f7f03ca169

commit 850d38c2fcc1a887aa0f38319eb1b8f7f03ca169
Author: mgiuca <mgiuca@chromium.org>
Date: Tue Oct 11 01:15:15 2016

Web Share: Remove dead code for showing a warning prompt in incognito.

This reverts commit 7db85418178d9d4f01a7901da7450e40e08414ec
(https://crrev.com/420564). The feature has previously been disabled, so
this change should not affect behaviour.

BUG= 645007 

Review-Url: https://codereview.chromium.org/2395963002
Cr-Commit-Position: refs/heads/master@{#424323}

[modify] https://crrev.com/850d38c2fcc1a887aa0f38319eb1b8f7f03ca169/chrome/android/java/src/org/chromium/chrome/browser/externalnav/ExternalNavigationDelegateImpl.java
[modify] https://crrev.com/850d38c2fcc1a887aa0f38319eb1b8f7f03ca169/chrome/android/java/src/org/chromium/chrome/browser/webshare/ShareServiceImpl.java

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e27d189dc63ecb32c2645828557389de11e7a466

commit e27d189dc63ecb32c2645828557389de11e7a466
Author: mgiuca <mgiuca@chromium.org>
Date: Thu Oct 06 23:41:46 2016

Web Share: Disable warning when web share is used from incognito.

This was deemed unnecessary in UX review. When navigator.share is used
from incognito, you still see the intent picker which is considered
sufficient warning for users.

The bulk of the code to support this (https://crrev.com/420564) remains
and will be removed in a future CL.

BUG= 645007 

Review-Url: https://codereview.chromium.org/2389393004
Cr-Commit-Position: refs/heads/master@{#423740}

[modify] https://crrev.com/e27d189dc63ecb32c2645828557389de11e7a466/chrome/android/java/src/org/chromium/chrome/browser/webshare/ShareServiceImpl.java

[Automated comment] removing mislabelled merge-merged-2840