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>

bench/BlurOccludedRRectBench.cpp

@@ -27,8 +27,6 @@ class BlurOccludedRRectBench : public Benchmark {
 
     void onDraw(int loops, SkCanvas* canvas) override {
         for (int l = 0; l < loops; ++l) {
-            canvas->clear(0xFFFAFAFA);
-
             SkPaint opaque;
             opaque.setAntiAlias(true);
             opaque.setColor(SK_ColorWHITE);

bench/ChartBench.cpp

@@ -132,9 +132,6 @@ class ChartBench : public Benchmark {
         }
 
         for (int frame = 0; frame < loops; ++frame) {
-
-            canvas->clear(0xFFE0F0E0);
-
             SkPath plotPath;
             SkPath fillPath;
 

bench/GMBench.cpp

@@ -27,6 +27,7 @@ void GMBench::onDraw(int loops, SkCanvas* canvas) {
     // 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);
     }
 }