SimpleOperationTest.ClearAndSwap/ES2_WGL failed when run in isolation,
since getGLWindow()->hasError() would report a previous error,
instead of result of swapBuffers().
When running after an OPENGL test, swapBuffers() would clear
the previous error, but that doesn't happen in isolation.

The previous error is from loading WGL functions, some of which are
expected not to be present. Clear the error in GetProcAddressWithFallback,
but verify that there is no error entering it.

This uncovers more errors in angle_perftests:
DrawCallPerfBenchmark.Run/wgl
DrawCallPerfBenchmark.Run/wgl_tex_change
DrawCallPerfBenchmark.Run/wgl_vbo_change
DrawElementsPerfBenchmark.Run/wgl_ushort
They come from redundant calls when destroying a window. Fix this as well.

Several more errors where uncovered by debug prints, fix those, too.

Bug: angleproject:3153
Change-Id: I559c098be9dcdfd3add83f045f745d190250b986
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1515602
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>