Revert "[omnibox] Clean up #omnibox-keyword-space-triggering feature"

This reverts commit a52e9d6a57b61d9bf9c24070931f4c45c32ebd70.

Reason for revert: The change prior to this broke tests, need to revert this so that can also be reverted.

The other revert: https://chromium-review.googlesource.com/c/chromium/src/+/2870512

Original change's description:
> [omnibox] Clean up #omnibox-keyword-space-triggering feature
>
> This CL removes the keyword space triggering feature flag.
> Double-space triggering is being deprecated and the option to disable
> space triggering is being replaced with a setting in the manage
> search engines page, gated by the #omnibox-keyword-trigger-setting flag
> (crrev.com/c/2848754).
>
> Bug: 1186107
> Change-Id: Id85775581cd20f0b85f01387598dd8f2035b0b45
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2847993
> Commit-Queue: Angela Yoeurng <[email protected]>
> Reviewed-by: Orin Jaworski <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#878698}

Bug: 1186107
Change-Id: I7a5393fcb8dc5dfd9fd45f1b0b01b549ef3366aa
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2870907
Auto-Submit: Timothy Loh <[email protected]>
Bot-Commit: Rubber Stamper <[email protected]>
Commit-Queue: Timothy Loh <[email protected]>
Owners-Override: Timothy Loh <[email protected]>
Cr-Commit-Position: refs/heads/master@{#878752}
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index 60c36f6a..0f23cf5 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -1712,6 +1712,12 @@
     "Enables an experimental scoring mode for suggestions when Google is the "
     "default search engine.";
 
+const char kOmniboxKeywordSpaceTriggeringName[] =
+    "Omnibox Keyword Space Triggering";
+const char kOmniboxKeywordSpaceTriggeringDescription[] =
+    "Controls whether keyword mode can be triggered by space, double space, or "
+    "neither.";
+
 const char kOmniboxKeywordSpaceTriggeringSettingName[] =
     "Omnibox Keyword Space Triggering Setting";
 const char kOmniboxKeywordSpaceTriggeringSettingDescription[] =