Add SkMatrix44 constructor that allows user to control the initialization

Add SkMatrix44 constructor that allows user to control the initialization
method.  Preliminary results using Chromium's cc_perftests
show that we can avoid almost half of the default constructors from
redundantly/unnecessarily initializing the matrix to identity.

Committed: https://code.google.com/p/skia/source/detail?r=6742

[shawnsingh, 2012-12-04 05:29:25]

PTAL - I'm new to skia land, so please feel free to nitpick as much as you like.

My intention is to let this type of new constructor roll into chromium, so that
we can remove all instances of default constructors in chromium and WebKit.  
Then, a second skia patch will make the default constructor private so it cannot
be used.

[reed1, 2012-12-05 14:18:28]

Perhaps we can make it easier for the compiler to totally ignore the constructor
in the case where the caller wants it to be uninitialized.

Here is an alternative style, not meant to land, but just so you can see the
difference. The names are long, and I'm up to hear a shorter suggestion.

https://codereview.appspot.com/6882043

[shawnsingh, 2012-12-05 18:23:50]

OK, cool. I like that idea a lot; I'll write the implementation and tests and
upload it here again, if that's good for you?

Also replied on that CL about default constructor.  Personally I'm against
re-introducing it.

[shawnsingh, 2012-12-08 00:15:27]

PTAL - New version with different enum per initialization type.

At this time, the kUnititialized_Constructor feature is what we need for
improving performance =)

About removing default constructor --> as per offline discussion and discussion
on https://codereview.appspot.com/6882043, I won't remove the default
constructor.  If you guys would like me to skip initialization on the default
constructor, I would be happy to do that in a separate patch (needs to happen
after modifying ui/gfx/transform in chrome first), just let me know if that's
what you want!

[reed1, 2012-12-10 13:53:01]

lgtm