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This CL prepares the C++ binding for passing exported passwords to the
Java code controlling Chrome's settings. This consists of:

(1) Extending the PasswordUIViewAndroid C++ class to handle a call to
start serialising passwords, to spawn a task to copy and serialize the
passwords on a backend task runner, and to post the result back to
Java.

(2) Teaching the Java class SavePasswordsPreferences to trigger the
serialisation when the user taps on the Export menu item.

The changes in (1) specifically make life-time management of
PasswordUIViewAndroid a little more complex: care is take that the
class always survives the backend processing. This includes
introducing a state variable and postponing self-destruction
in Destroy().

The CL also adds a whole new unittest for PasswordUIViewAndroid and
one more scenario to the existing SavePasswordsPreferencesTest. The
code in either language is tested, but passing the information through
JNI is not, because it is not clear how that would be done.

The format changes to BUILD.gn (beyond introducing the new unittest)
are unrelated, but a result of the compulsory running of git cl format.

The CL does not introduce any processing of the serialised data in the
Java code yet (it is currently dropped on receipt). That will come in
future CLs.

Bug: 788701
Change-Id: I32bbcb4bf62883d3154b3abf81ea7b076c05e37c
Reviewed-on: https://chromium-review.googlesource.com/809127
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530603}