Fix benches that aren't really timing what we want

Remove full clears from benches
Fix unbounded canvas modification in GM benches

Bug: skia:
Change-Id: Ie3d67282714a7b4e980aec399056c0e9df330993
Reviewed-on: https://skia-review.googlesource.com/86040
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/bench/GMBench.cpp b/bench/GMBench.cpp
index 6037a82..297ff42 100644
--- a/bench/GMBench.cpp
+++ b/bench/GMBench.cpp
@@ -27,6 +27,7 @@
     // Does the GM ever rely on drawBackground to lazily compute something?
     fGM->drawBackground(canvas);
     for (int i = 0; i < loops; ++i) {
+        SkAutoCanvasRestore acr(canvas, true);
         fGM->drawContent(canvas);
     }
 }