Fix DesktopCapturer constructors to handle failures properly.
Fix DesktopCapturer constructors to handle failures properly.

CreateRawScreenCapturer() and CreateRawWindowCapturer() in
DesktopCapturer are allowed to return nullptr when capturer cannot be
initialized for some reason. CreateWindowCapturer() and
CreateScreenCapturer() in DesktopCapturer were not handling this case
correctly, which may lead to crash.

BUG=chromium:702745

Review-Url: https://codereview.webrtc.org/2754403002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#17298}

ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc