Move GrContextPriv methods to their own .cpp file

Centralize these for my sanity. Most will also be parceled out to other contexts.

Change-Id: If0e7e98bcf66c4d8a3391f9b04e643ccc91af4ad
Reviewed-on: https://skia-review.googlesource.com/c/189488
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/samplecode/SampleChineseFling.cpp b/samplecode/SampleChineseFling.cpp
index 84181aa..b123942 100644
--- a/samplecode/SampleChineseFling.cpp
+++ b/samplecode/SampleChineseFling.cpp
@@ -160,19 +160,19 @@
             GrContext* grContext = canvas->getGrContext();
             if (grContext) {
                 sk_sp<SkImage> image =
-                grContext->priv().getFontAtlasImage_ForTesting(
+                grContext->priv().testingOnly_getFontAtlasImage(
                                                             GrMaskFormat::kA8_GrMaskFormat, 0);
                 canvas->drawImageRect(image,
                                       SkRect::MakeXYWH(10.0f, 10.0f, 512.0f, 512.0), &paint);
-                image = grContext->priv().getFontAtlasImage_ForTesting(
+                image = grContext->priv().testingOnly_getFontAtlasImage(
                                                             GrMaskFormat::kA8_GrMaskFormat, 1);
                 canvas->drawImageRect(image,
                                       SkRect::MakeXYWH(522.0f, 10.0f, 512.f, 512.0f), &paint);
-                image = grContext->priv().getFontAtlasImage_ForTesting(
+                image = grContext->priv().testingOnly_getFontAtlasImage(
                                                             GrMaskFormat::kA8_GrMaskFormat, 2);
                 canvas->drawImageRect(image,
                                       SkRect::MakeXYWH(10.0f, 522.0f, 512.0f, 512.0f), &paint);
-                image = grContext->priv().getFontAtlasImage_ForTesting(
+                image = grContext->priv().testingOnly_getFontAtlasImage(
                                                             GrMaskFormat::kA8_GrMaskFormat, 3);
                 canvas->drawImageRect(image,
                                       SkRect::MakeXYWH(522.0f, 522.0f, 512.0f, 512.0f), &paint);