Switch tough_compositor_cases tests to use will-change transform

In crbug.com/1012775 we are unifying how trivial 3d transforms are
handled across platforms by making them not a compositing hint. This
caused performance regressions on tough_compositor_cases which were
using trivial 3d transforms as a compositing hint.

This patch updates tough_compositor_cases to be more explicit about
layer triggers by using "will-change: transform".

Bug: 1012775, 1016320
Change-Id: I1734e885ec287d1ca608f7a70674b8b0f74b6b36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872177
Commit-Queue: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Auto-Submit: Philip Rogers <pdr@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707913}

chrome/test/data/perf/tough_compositor_cases/css_opacity_plus_n_layers.html

@@ -15,7 +15,7 @@ found in the LICENSE file.
   float:left;
   background: blue;
   margin: 2px;
-  -webkit-transform: translateZ(0)
+  will-change: transform;
 }
 
 .quad > p {

chrome/test/data/perf/tough_compositor_cases/infinite_scroll_element_n_layers.html

@@ -30,6 +30,7 @@ found in the LICENSE file.
         contain: layout;
       }
       .item {
+        will-change: transform;
         -webkit-transform: translate3d(0,0,0);
         box-sizing: border-box;
         overflow: hidden;

chrome/test/data/perf/tough_compositor_cases/infinite_scroll_root_fixed_n_layers.html

@@ -28,6 +28,7 @@ found in the LICENSE file.
         contain: layout;
       }
       .item {
+        will-change: transform;
         -webkit-transform: translate3d(0,0,0);
         box-sizing: border-box;
         overflow: hidden;

chrome/test/data/perf/tough_compositor_cases/infinite_scroll_root_n_layers.html

@@ -28,6 +28,7 @@ found in the LICENSE file.
         contain: layout;
       }
       .item {
+        will-change: transform;
         -webkit-transform: translate3d(0,0,0);
         box-sizing: border-box;
         overflow: hidden;

chrome/test/data/perf/tough_compositor_cases/js_opacity_plus_n_layers.html

@@ -15,7 +15,7 @@ found in the LICENSE file.
   float:left;
   background: blue;
   margin: 2px;
-  -webkit-transform: translateZ(0)
+  will-change: transform;
 }
 
 .quad > p {

chrome/test/data/perf/tough_compositor_cases/js_paint_plus_n_layers.html

@@ -15,7 +15,7 @@ found in the LICENSE file.
   float:left;
   background: blue;
   margin: 2px;
-  -webkit-transform: translateZ(0)
+  will-change: transform;
 }
 
 .quad > p {