Add clear function to TestAtlasTextRendering interface

Bug: skia:
Change-Id: I24563abe5e72e57e6764ddf6aca76ccf098cc488
Reviewed-on: https://skia-review.googlesource.com/73640
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/gm/atlastext.cpp b/gm/atlastext.cpp
index b8acb0f..a25bb87 100644
--- a/gm/atlastext.cpp
+++ b/gm/atlastext.cpp
@@ -64,6 +64,7 @@
             canvas->clear(SK_ColorRED);
             return;
         }
+        fRenderer->clearTarget(fTarget->handle(), 0xFF808080);
         auto bmp = this->drawText();
         SkPaint paint;
         paint.setBlendMode(SkBlendMode::kSrc);