Add field trial testing config for BoostThreadPriorityOnLibraryLoadingAndBackgroundMode.

This experiment enables:
- Use THREAD_MODE_BACKGROUND_BEGIN/END to set the priority of
  background threads. This affects memory and IO priority in addition
  to CPU priority.
- Increase thread priority to "normal" in scopes where background
  threads load modules.

The expectation is that this will improve responsiveness of Chrome
(input delay, first contentful paint), without regressing jank.

We are adding the testing config in preparation for enabling this
via Finch in M79.

Bug: 973868
Change-Id: I467cae12503e3ad724b665f3aebdaeb94e1325c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1835023
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Auto-Submit: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705960}

testing/variations/fieldtrial_testing_config.json

@@ -1434,6 +1434,22 @@
             ]
         }
     ],
+    "BoostThreadPriorityOnLibraryLoadingAndBackgroundMode": [
+        {
+            "platforms": [
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "BoostPriorityAndBackgroundThreadMode",
+                    "enable_features": [
+                        "BoostThreadPriorityOnLibraryLoading",
+                        "WindowsThreadModeBackground"
+                    ]
+                }
+            ]
+        }
+    ],
     "BrowserBlacklist": [
         {
             "platforms": [