predictors: Add an about:flags flag for speculative prefetch.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 1a58b6fd7b6b3a72ebc3e29a26a705d5d9d0ae64..eb5f09b6427261678726ff5c8564e018229d769d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -98631,6 +98631,7 @@ from previous Chrome versions.
   <int value="-1943507605" label="enable-new-video-renderer"/>
   <int value="-1941852572" label="floating-virtual-keyboard"/>
   <int value="-1940806558" label="enable-syncfs-directory-operation"/>
+  <int value="-1940291343" label="SpeculativeResourcePrefetching:enabled"/>
   <int value="-1938263248" label="enable-extension-info-dialog"/>
   <int value="-1937077699" label="http-form-warning"/>
   <int value="-1934673791" label="gl-composited-texture-quad-border"/>
@@ -98843,6 +98844,7 @@ from previous Chrome versions.
   <int value="-1075156797" label="enable-brotli"/>
   <int value="-1075089382" label="enable-physical-web"/>
   <int value="-1073479583" label="ShowArcFilesApp:disabled"/>
+  <int value="-1067635248" label="SpeculativeResourcePrefetching:disabled"/>
   <int value="-1062119671" label="enable-password-force-saving"/>
   <int value="-1056310158" label="disable-memory-pressure-chromeos"/>
   <int value="-1052782474" label="enable-cloud-devices"/>