ScreenCapturer tests may fail on trybot, so this change is to fix the issue.
Changes include,
1. Sometimes, a capturer may capture part of the change, i.e. usually the draw
actions are not atomic. So the updated_region may be inaccurate. So I have added
a MayDrawIncompleteShapes() function in ScreenDrawer. If it returns false, the
updated_region check will be ignored.
2. Several test cases may run concurrently, which makes one ScreenDrawer won't
really work. Its window may be covered by another ScreenDrawer. So I have added
a system wide lock to ensure only one ScreenDrawer is working at a certain time.
3. On unity (Linux), the top several pixels of a window may be covered by a
shadow effect if the window is not focused. So I have added a BringToFront()
function, and call it in WaitForPendingDraws().
4. On Windows, the drawn shapes are 'temporary drawing', which will be erased
once the window is covered by another one. So I repeat DrawRectangle() function
call in the test case.
TODO(zijiehe): The DISABLED_ prefixes will be added back after the code review.
And I will move these test cases into modules_test in a coming change.
BUG=647067
Review-Url: https://codereview.webrtc.org/2337073007
Cr-Commit-Position: refs/heads/master@{#14674}
