Reland "Create field trial for hover card images."

This reverts commit cb8fbfbf.

Reason for revert: 3P file had fallen out of TSAN whitelist when it was renamed, it has since been added back.

See: https://chromium-review.googlesource.com/c/chromium/src/+/2076278

Original change's description:
> Revert "Create field trial for hover card images."
>
> This reverts commit 9ba868cd.
>
> Reason for revert: <INSERT REASONING HERE>
> interactive_ui_tests failed on
> Builder Linux TSan Tests
> https://ci.chromium.org/p/chromium/builders/ci/Linux%20TSan%20Tests
>
> Original change's description:
> > Create field trial for hover card images.
> >
> > This is required to deploy a finch configuration.
> > See cr/296239960 for WIP Finch config.
> >
> > Bug: 1032640
> > Change-Id: Ifae3d2966d4ca908982cec37c5d88f786bf07367
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067558
> > Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
> > Commit-Queue: Dana Fried <dfried@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#743826}
>
> TBR=rkaplow@chromium.org,dfried@chromium.org
>
> Change-Id: I27fbe4d67b42a42d63757e78427d86d31e045592
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 1032640
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2069275
> Reviewed-by: Lan Wei <lanwei@chromium.org>
> Commit-Queue: Lan Wei <lanwei@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#743839}

TBR=rkaplow@chromium.org,lanwei@chromium.org,dfried@chromium.org

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

Bug: 1032640
Change-Id: I13c1920b2e3d0a721af5475df789b15cf0794904
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2091933
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Dana Fried <dfried@chromium.org>
Reviewed-by: Lan Wei <lanwei@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747931}

testing/variations/fieldtrial_testing_config.json

@@ -5401,6 +5401,24 @@
             ]
         }
     ],
+    "TabHoverCardImages": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "TabHoverCardImages"
+                    ]
+                }
+            ]
+        }
+    ],
     "TabHoverCards": [
         {
             "platforms": [