Reland "Prepare CSSOMViewScrollCoordinates for finch trial"

This is a reland of da0cc146

Original change's description:
> Prepare CSSOMViewScrollCoordinates for finch trial
>
> Following best practice advice from go/finch-blink:
>  - Update testing config variation to enable the flag
>  - Update file_manager test to expect the new behavior
>
> Bug:721759
>
> Change-Id: I727eef6b50cf09027945bcd645335b39ee83ba2d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087422
> Commit-Queue: Majid Valipour <majidvp@chromium.org>
> Reviewed-by: Noel Gordon <noel@chromium.org>
> Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
> Reviewed-by: Robert Flack <flackr@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#749856}

TBR: flackr@chromium,noel@chromium.org
Bug: 721759
Change-Id: Iaf9d9803e2baa2032de9335432fb5442cc8be4e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2101188Reviewed-by: Majid Valipour <majidvp@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750193}

testing/variations/fieldtrial_testing_config.json

@@ -1185,6 +1185,26 @@
             ]
         }
     ],
+    "CSSOMViewScrollCoordinates": [
+        {
+            "platforms": [
+                "android",
+                "android_webview",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "EnableCSSOMViewScrollCoordinates",
+                    "enable_features": [
+                        "CSSOMViewScrollCoordinates"
+                    ]
+                }
+            ]
+        }
+    ],
     "CacheStorageEagerReading": [
         {
             "platforms": [

ui/file_manager/integration_tests/file_manager/directory_tree.js

@@ -312,13 +312,11 @@
     chrome.test.assertTrue(await remoteCall.callRemoteTestUtil(
         'requestAnimationFrame', appId, []));
 
-    // Get the scroll limits: see crbug.com/721759 for RTL |scrollRight|.
     const scrolled = await remoteCall.waitForElementStyles(
         appId, directoryTree, ['scrollLeft']);
-    const scrollRight = scrolled.scrollWidth - scrolled.renderedWidth;
 
     // Check: the directory tree should not be horizontally scrolled.
-    const notScrolled = scrolled.scrollLeft === scrollRight;
+    const notScrolled = scrolled.scrollLeft === 0;
     chrome.test.assertTrue(notScrolled, 'Tree should not scroll right');
   };