Reland "Add field trial testing config for BoostThreadPriorityOnLibraryLoadingAndBackgroundMode."

This is a reland of 9d6a01ba
We now handle negative values returned from ::GetThreadPriority().
https://chromium-review.googlesource.com/c/chromium/src/+/1829570

Original change's description:
> 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}

TBR=rkaplow@chromium.org

Bug: 973868
Change-Id: I392cfb2f92e2ac0fa888fa1e323557ab35055b48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872312Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707822}

testing/variations/fieldtrial_testing_config.json

@@ -1419,6 +1419,22 @@
             ]
         }
     ],
+    "BoostThreadPriorityOnLibraryLoadingAndBackgroundMode": [
+        {
+            "platforms": [
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "BoostPriorityAndBackgroundThreadMode",
+                    "enable_features": [
+                        "BoostThreadPriorityOnLibraryLoading",
+                        "WindowsThreadModeBackground"
+                    ]
+                }
+            ]
+        }
+    ],
     "BrowserBlacklist": [
         {
             "platforms": [