Add new pattern for creating downstream objects

Add new pattern for creating downstream objects

This change introduces a new pattern for instantiating objects of
classes that have have differing upstream/downstream implementations.
If a class's constructor requires no parameters (as is most frequently
the case in this scenario), it can be created with:

    ChromeApplication.createObject(MyType.class);

and registered with no additional change upstream.  Only a single line
registration of the implementation needs to be provided downstream.
Without this change, one must register a new method both upstream and
downstream as well as cast an application context to a
ChromeApplication before calling that newly registered return
type-specifc method.

BUG=685383

[cco3, 2017-02-01 02:30:59]

cco3@chromium.org changed reviewers:
+ mariakhomenko@chromium.org

[cco3, 2017-02-01 02:31:00]

I think this will work better.  Sorry for the run-around.

[Maria, 2017-02-01 03:49:09]

lgtm

[cco3, 2017-02-15 23:23:01]

Description was changed from

==========
Use a method to get app-specific implementations

Using a protected map to get app-specific implementations created an
uncertainty between upstream and downstream code regarding whether the
map has been properly initialized.  Using a getImplementation() method
will let the base class and subclass of ChromeApplication do whatever
initialization and verification they would like to do independently.

BUG=685383,687142
==========

to

==========
Add new pattern for creating downstream objects

This change introduces a new pattern for instantiating objects of
classes that have have differing upstream/downstream implementations.
If a class's constructor requires no parameters (as is most frequently
the case in this scenario), it can be created with:

    ChromeApplication.createObject(MyType.class);

and registered with no additional change upstream.  Only a single line
registration of the implementation needs to be provided downstream.
Without this change, one must register a new method both upstream and
downstream as well as cast an application context to a
ChromeApplication before calling that newly registered return
type-specifc method.

BUG=685383
==========