[StartSurface] Add a trendy-terms variation

Trendy-terms is like omnibox-only, with the only difference that there
is a horizontal RecyclerView below the omnibox that contains clickable
trendy terms. Note that since this component is not yet hooked with
data source, the trendy-terms variation looks the same as omnibox-only
right now. This CL only introduces the components to contain the
trendy terms that come later.

Bug: 1099037
Change-Id: I579589123efc275d62d14f0c202f575d2609ce6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2265236
Reviewed-by: Theresa  <[email protected]>
Reviewed-by: Steven Holte <[email protected]>
Reviewed-by: Wei-Yin Chen (陳威尹) <[email protected]>
Commit-Queue: Yue Zhang <[email protected]>
Cr-Commit-Position: refs/heads/master@{#782662}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 79c2c4b..d2b7363b 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1406,6 +1406,10 @@
     {"omnibox_scroll_mode", "pinned"},
     {"hide_switch_when_no_incognito_tabs", "true"}};
 
+const FeatureEntry::FeatureParam kStartSurfaceAndroid_TrendyTerms[] = {
+    {"start_surface_variation", "trendyterms"},
+    {"hide_switch_when_no_incognito_tabs", "true"}};
+
 const FeatureEntry::FeatureVariation kStartSurfaceAndroidVariations[] = {
     {"Single Surface", kStartSurfaceAndroid_SingleSurface,
      base::size(kStartSurfaceAndroid_SingleSurface), nullptr},
@@ -1432,7 +1436,9 @@
     {"Omnibox Only, Quick", kStartSurfaceAndroid_OmniboxOnly_Quick,
      base::size(kStartSurfaceAndroid_OmniboxOnly_Quick), nullptr},
     {"Omnibox Only, Pinned", kStartSurfaceAndroid_OmniboxOnly_Pinned,
-     base::size(kStartSurfaceAndroid_OmniboxOnly_Pinned), nullptr}};
+     base::size(kStartSurfaceAndroid_OmniboxOnly_Pinned), nullptr},
+    {"Trendy Terms", kStartSurfaceAndroid_TrendyTerms,
+     base::size(kStartSurfaceAndroid_TrendyTerms), nullptr}};
 
 const FeatureEntry::FeatureParam kConditionalTabStripAndroid_Immediate[] = {
     {"conditional_tab_strip_session_time_ms", "0"}};