fix: crash when invoking login callback synchronously

[nornagon]

Description of Change

Closes #30065.

This is because somehow the LoginHandler is deleted while calling Emit(), so
accessing the auth_required_callback_ member variable after that is a
segmentation fault.

It looks like calling cb() from JS ultimately ends up deleting the
LoginHandler:
https://source.chromium.org/chromium/chromium/src/+/main:content/browser/storage_partition_impl.cc;l=523;drc=ded7717f5252a0c9d9de56bc22483b195d1801e0.

So this crashes:

electron/shell/browser/login_handler.cc

Line 73 in ccfde6c

if (!default_prevented && auth_required_callback_) {

because this has been deleted.

We can work around that by taking a weak ptr before the Emit call and checking it afterwards, to guard against deletion. In that situation, auth_required_callback_ would have been null anyway, so there shouldn't be any behavior change.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Fixed a crash when calling the webContents.on('login') callback synchronously.

[nornagon]

test failures are unrelated; merging.

[release-clerk]

Release Notes Persisted

Fixed a crash when calling the webContents.on('login') callback synchronously.

[trop]

I was unable to backport this PR to "13-x-y" cleanly;
you will need to perform this backport manually.

[trop]

I was unable to backport this PR to "12-x-y" cleanly;
you will need to perform this backport manually.

[trop]

I have automatically backported this PR to "14-x-y", please check out #30090

[trop]

@nornagon has manually backported this PR to "13-x-y", please check out #30091

[trop]

@nornagon has manually backported this PR to "12-x-y", please check out #30092