Bug 1611374 Comment 3 Edit History

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Chrome does not allow nested `Document.execCommand()` calls:
https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/renderer/core/editing/commands/document_exec_command.cc;l=75;drc=301e5d079a1b4c29c5b17574d0470e6db7370acc

On the other hand, Safari (and Firefox) allows it.  However, it's worthwhile to
follow Chrome's behavior because our editor security issues are usually found
with nested `execCommand` calls.

This patch makes `Document::ExecCommand()` return `false` when it's called
while running another `Document::ExecCommand()` call on Nightly and early Beta.

And this patch sets the pref to `true` when all crash tests under
`editor/libeditor/crashtests` which depend on nested calls of `execCommand` run
since same things may be reproducible with other DOM APIs.
Chrome does not allow nested `Document.execCommand()` calls:
https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/renderer/core/editing/commands/document_exec_command.cc;l=75;drc=301e5d079a1b4c29c5b17574d0470e6db7370acc

On the other hand, Safari (and Firefox) allows it.  However, it's worthwhile to
follow Chrome's behavior.

This patch makes `Document::ExecCommand()` return `false` when it's called
while running another `Document::ExecCommand()` call on Nightly and early Beta.
This is exactly same behavior, and we should watch broken web apps reports
for a while before riding this on the train.

And this patch sets the pref to `true` when all crash tests under
`editor/libeditor/crashtests` which depend on nested calls of `execCommand` run
since same things may be reproducible with other DOM APIs.

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