M35: Move to 35.0.1912.2.
M35 gardening.

base::NativeLibrary now takes a base::NativeLibraryLoadError as second
parameter instead of a std::string.

CL: https://codereview.chromium.org/206713004
M35 gardening.

Adjust Runtime::WebContentsCreated's signature.

https://codereview.chromium.org/207353003
M35 gardening.

Adapt RuntimeNetworkDelegate::OnHeadersReceived's signature.

https://codereview.chromium.org/154473002
M35 gardening.

Adapt to changes to how ContentSuiteTestBase works.

https://codereview.chromium.org/190663012
M35 Gardening: Do not initialize XWalkRunner twice

This is all being created already when we create a XWalkMainDelegate
at XWalkTestLauncherDelegate.

Reflects the simplification landed here:
https://codereview.chromium.org/190663012