Add new Smart Dim model behind new flag kSmartDim20190221

The flag controls
* Which feature-preprocessor config to use
-- Added new feature preprocessor
* Which ML Service model to load
-- ML Service model previously added in crrev.com/c/1614832
* Default dim_threshold parameter

Added a UI toggle for the flag in chrome://flags.

Testing notes:
I pushed this to CrOS device and manually invoked the model (with and
without the flag enabled). I invoked the model using the Smart Dim D-Bus
interface added in under-review CL crrev.com/c/1598921. That CL also
unblocks adding a Tast integration test that can exercise both models.

Bug: 937063
Test: See above.
Change-Id: Id085b5caf9cb73a26febd6c154e9110561a48f2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626836
Commit-Queue: Andrew Moylan <[email protected]>
Reviewed-by: Jia Meng <[email protected]>
Reviewed-by: calamity <[email protected]>
Cr-Commit-Position: refs/heads/master@{#663481}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 86382840..d96bf6c 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1871,6 +1871,9 @@
 extern const char kShowTouchHudName[];
 extern const char kShowTouchHudDescription[];
 
+extern const char kSmartDim20190221Name[];
+extern const char kSmartDim20190221Description[];
+
 extern const char kSmartTextSelectionName[];
 extern const char kSmartTextSelectionDescription[];