ANGLETest provides useful functionality for end2end tests, but it
cannot be used for tests which take other test parameters than the
platform. The patch introduces another level of abstraction by moving
all of ANGLETest's functionality to another class - ANGLETestBase -
which does not inherit from ::testing::TestWithParam<>. New tests can
either inherit from ANGLETest as before to have only platform
parameters, or they can inherit from ANGLETestBase and add handling
of custom test parameters.

Example:

// The new parameter type must inherit from PlatformParameters.
struct MyCustomParameters : PlatformParameters
{
    bool mWorkaroundState;
};

class MyTest : public ANGLETestBase,
               public ::testing::TestWithParam<MyCustomParameters>
{
    protected:
    void overrideWorkaroundsD3D(
        angle::WorkaroundsD3D *workaroundsD3D) override
    {
        workaroundsD3D->myCustomWorkaround = GetParam().mWorkaroundState;
    }

};

ANGLE_INSTANTIATE_TEST(MyTest, MyCustomParameters(ES3_D3D11(), false),
                               MyCustomParameters(ES3_D3D11(), true));

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: Ia36e429cff8c4c291fc87a286a1d1a3004d6fad6
Reviewed-on: https://chromium-review.googlesource.com/530945
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>