Generate list of GPU contexts outside SurfaceTest tests

Add support for feeding the tests with contexts directly to the unit
test framework.

This fixes the problem where tests are more complex than needed just in
order to run the test code with multiple backends.

Also makes it possible to change the logic how contexts are
created. Instead of direct numbering, the different testable contexts
may be generated from filtered cross-product of context options. For
example: currently NVPR is a type of context. However, it could be also
an on/off feature of any context. In order to test this kind of context,
the enumeration can not be just of context type. It's simpler
to move the enumeration out of the tests.

A test targeting both normal and GPU backends would look like:

static void test_obj_behavior(skiatest::Reporter* reporter,
    SkObj* obj, [other params] ) {
    ... test with obj and param ..
}
DEF_TEST(ObjBehavior, reporter) {
    for (auto& object : generate_object) {
        for (auto& other_param : generate_other_variant) {
	   test_obj_behavior(reporter, object, other_param);
	}
    }
}
#if SK_SUPPORT_GPU
DEF_GPUTEST_FOR_ALL_CONTEXTS(ObjBehavior_Gpu, reporter, context) {
    for (auto& object : generate_gpu_object) {
        for (auto& other_param : generate_other_variant) {
	   test_obj_behavior(reporter, object, other_param);
	}
    }
}
#endif

Uses the feature in SurfaceTests as an example.

Moves SkSurface -related tests from ImageTest to SurfaceTest.

BUG=skia:2992

Review URL: https://codereview.chromium.org/1446453003
7 files changed