Switch Win XR perf to x64 browser

Switches the browser used by the Windows XR perf tests from "release" to
"release_x64" since the bot was changed to 64-bit and Telemetry is
now failing to find the 32-bit browser that is requested.

TBR=ynovikov@chromium.org

Bug: 988833
Change-Id: I498e65f1ac4182718ccf83495d6ff1eb3eacbe73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1735553Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684078}

testing/buildbot/chromium.gpu.fyi.json

@@ -30573,7 +30573,7 @@
           "-v",
           "--upload-results",
           "--output-format=histograms",
-          "--browser=release"
+          "--browser=release_x64"
         ],
         "experiment_percentage": 100,
         "isolate_name": "vr_perf_tests",

testing/buildbot/test_suites.pyl

@@ -4799,7 +4799,7 @@
           '-v',
           '--upload-results',
           '--output-format=histograms',
-          '--browser=release',
+          '--browser=release_x64',
         ],
         'isolate_name': 'vr_perf_tests',
         'merge': {