Refactor BlockDifference() to use MediaInitializer().

Refactor BlockDifference() to use MediaInitializer().

BUG=224662
TEST=compiles. unit tests.

[scherkus (not reviewing), 2013-06-05 00:00:47]

lgtm

[commit-bot: I haz the power, 2013-06-05 18:51:47]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/dalecurtis@chromium.org/16252006/4001

[commit-bot: I haz the power, 2013-06-05 20:01:06]

Sorry for I got bad news for ya.
Compile failed with a clobber build on android_clang_dbg.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=android_cl...
Your code is likely broken or HEAD is junk. Please ensure your
code is not broken then alert the build sheriffs.
Look at the try server FAQ for more details.

[DaleCurtis, 2013-06-05 20:25:16]

PTAL.  Needed a .gyp change to add a define since the conditions for screen
capture support are a bit complicated.

[scherkus (not reviewing), 2013-06-05 20:37:47]

lgtm

[commit-bot: I haz the power, 2013-06-05 20:40:44]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/dalecurtis@chromium.org/16252006/19001

[DaleCurtis, 2013-06-06 00:00:29]

Abandoned since https://codereview.chromium.org/15692018/ deletes this code.

[Sergey Ulanov, 2013-06-06 00:00:48]

I suspect this may break chromoting host because it never calls
InitializeMediaLibrary().
In either case this CL will be obsolete when I land
https://codereview.chromium.org/15692018/, so maybe we don't need it at all.

[DaleCurtis, 2013-06-06 00:01:56]

chromoting host calls it now: https://chromiumcodereview.appspot.com/15713012