dirty region than the real screen change. A similar behavior may happen on other
platforms with damage notification support. So it's better to have an individual
layer to handle the Differ logic, and remove capturing independent logic out of
each ScreenCapturer* implementation.
So this change does following things,
1. Update differ_block to handle variable height. differ_block_sse2 has been
renamed to differ_vector_sse2.
2. A new ScreenCapturerDifferWrapper implementation to help set
DesktopFrame::updated_region(). It uses an underlying ScreenCapturer to do
the real capture work, and updates the updated region of DesktopFrame returned
from OnCaptureResult function.
3. FakeDesktopCapturer and FakeScreenCapturer to generate controllable
DesktopFrame by using DesktopFrameGenerator and DesktopFramePainter.
4. Test ScreenCapturerDifferWrapper by using FakeScreenCapturer.
After this change, we can eventually remove all Differ logic from
ScreenCapturer* implementations, and fix a potential crash bug in
ScreenCapturerLinux class. It wrongly assumes previous_frame() has a same size
as current_frame(). https://goo.gl/3nSqOC
BUG=633802
TBR=kjellander@webrtc.org
Review-Url: https://codereview.webrtc.org/2202443002
Cr-Commit-Position: refs/heads/master@{#14076}
