[FastTextAutosizer] Enable FastTextAutosizer on tot/dev by default

This patch enables the FastTextAutosizer on tot/dev by default in order
to get some early feedback before flipping this live everywhere.

I've added a tri-state default/enabled/disabled flag in about://flags
where the disabled state takes precedence. By default, chrome
now has the fast text autosizer enabled but content does not.

BUG=352373

Review URL: https://codereview.chromium.org/203473003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@258661 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 5a77fbba..b98d001 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1766,7 +1766,8 @@
     IDS_FLAGS_ENABLE_FAST_TEXT_AUTOSIZING_NAME,
     IDS_FLAGS_ENABLE_FAST_TEXT_AUTOSIZING_DESCRIPTION,
     kOsAndroid,
-    SINGLE_VALUE_TYPE(switches::kEnableFastTextAutosizing)
+    ENABLE_DISABLE_VALUE_TYPE(switches::kEnableFastTextAutosizing,
+                              switches::kDisableFastTextAutosizing)
   },
   {
     "disable-cast",