Fix for WebView not drawing in a layer

internal_private_accessTopLayerRenderTargetContext does not
guarantee that a GrRenderTarget is created. Moving flush before
internal_private_accessTopLayerRenderTargetContext resolves that
issue. GrRenderTarget was often nullptr, when drawing WebView in a
layer.

Test: Passed CtsUiRenderingTestCases. Ran two tests apps attached to the bug
Bug: 156413480
Change-Id: Ib3e23c4ab58eb3d02daa05979f545b75a8bfea07