media/.../{android_,gpu_}video_encode_accelerator{,_host} cleanup

Just found a couple of things that needed cleanup during
the investigation of the bug below. So, this CL:

- Removes unnecessary CHECK(env); after
 JNIEnv* env = AttachCurrentThread();
 (see https://crrev.com/2231923002)

- s/__PRETTY_FUNCTION__/__FUNCTION__/ because the former
 is illegible in adb logcat or any other log, really.

- s/NULL/nullptr/

And concretely in GpuVideoEncodeAccelerator this CL:

- moves the static methods
  std::unique_ptr<VideoEncodeAccelerator> Create{platform}VEA
 out of the class and into anonymous namespace of the .cc
 file, since they don't need to be in the class at all.

- nukes (*CreateVEAFp)() in that class and uses instead a
 base::Callback(), bound to the static method mentioned above.

- uses for-range loops.

BUG=638664
TEST=all unittests. content_browsertests, browser_tests etc
working, and tested by hand in N7.

Review-Url: https://codereview.chromium.org/2251993004
Cr-Commit-Position: refs/heads/master@{#413057}
5 files changed