Reland "Adding fieldtrial testing config for FastBorderRadius"

This reverts commit 30d10c99.

Reason for revert: Fixed underlying bug; see crbug.com/968255.

Original change's description:
> Revert "Adding fieldtrial testing config for FastBorderRadius"
> 
> This reverts commit 987deaf1.
> 
> Reason for revert: crash on browser_tests in linux-chromeos-dbg (see crbug.com/968255)
> 
> Original change's description:
> > Adding fieldtrial testing config for FastBorderRadius
> > 
> > This patch adds a field trial testing config for FastBorderRadius.
> > 
> > Bug: 947715
> > Change-Id: Iff94fdce62a195ead91bf338b40df0380a9aab5d
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632829
> > Commit-Queue: Robert Kaplow (slow) <rkaplow@chromium.org>
> > Auto-Submit: Mason Freed <masonfreed@chromium.org>
> > Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#664252}
> 
> TBR=rkaplow@chromium.org,masonfreed@chromium.org
> 
> Change-Id: I1f36b1b44df65c43e09cab18f238bc0598985146
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 947715, 968255
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1634424
> Reviewed-by: Jun Mukai <mukai@chromium.org>
> Commit-Queue: Jun Mukai <mukai@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#664424}

TBR=mukai@chromium.org,rkaplow@chromium.org,masonfreed@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 947715, 968255
Change-Id: Iecb9ee718680d3c751402da2a2e2773cc1736026
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1637871Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665139}

testing/variations/fieldtrial_testing_config.json

@@ -2165,6 +2165,25 @@
             ]
         }
     ],
+    "FastBorderRadius": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "FastBorderRadius"
+                    ]
+                }
+            ]
+        }
+    ],
     "FilterAdsOnAbusiveSites": [
         {
             "platforms": [