Add a flag for enabling Chrome Web Store Gallery widget app

This enables 'Printer detected' notification, clicking on which launches
Chrome Web Store widget  app showing list of apps that support the
detected printer.

BUG=439448

Review URL: https://codereview.chromium.org/1123143004

Cr-Commit-Position: refs/heads/master@{#330112}

chrome/app/chromeos_strings.grdp

@@ -6362,5 +6362,11 @@ All users must sign out to continue.
   <message name="IDS_WEBSTORE_WIDGET_DEFAULT_ERROR" desc="Default message shown when Chrome Web Store Widget app window content fails to load (e.g. if it fails to load Chrome Web Store Gallery in a webview).">
     Could not connect to the Chrome Web Store.
   </message>
+  <message name="IDS_FLAGS_PRINTER_PROVIDER_SEARCH_APP_NAME" desc="Name of the flag used to enable launching Chrome Web Store Gallery widget app for searching for printer provider apps.">
+    Chrome Web Store Gallery app for printer drivers
+  </message>
+  <message name="IDS_FLAGS_PRINTER_PROVIDER_SEARCH_APP_DESCRIPTION" desc="Description of a flag in chrome://flags that enables launching Chrome Web Store Gallery widget app for searching for printer provider apps.">
+    Enables Chrome Web Store Gallery app for printer drivers. The app searches Chrome Web Store for extensions that support printing to a USB printer with specific USB ID.
+  </message>
 
 </grit-part>

chrome/browser/about_flags.cc

@@ -2404,6 +2404,15 @@ const Experiment kExperiments[] = {
     kOsAll,
     SINGLE_VALUE_TYPE(switches::kDisableNewVideoRenderer)
   },
+#if defined(OS_CHROMEOS)
+  {
+    "enable-printer-app-search",
+    IDS_FLAGS_PRINTER_PROVIDER_SEARCH_APP_NAME,
+    IDS_FLAGS_PRINTER_PROVIDER_SEARCH_APP_DESCRIPTION,
+    kOsCrOS,
+    SINGLE_VALUE_TYPE(chromeos::switches::kEnablePrinterAppSearch)
+  },
+#endif  // OS_CHROMEOS
   // Temporary flag to ease the transition to standard-compliant scrollTop
   // behavior.  Will be removed shortly after http://crbug.com/157855 ships.
   {

tools/metrics/histograms/histograms.xml

@@ -56622,6 +56622,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="-649956990" label="enable-harfbuzz-rendertext"/>
   <int value="-641719457" label="disable-compositor-touch-hit-testing"/>
   <int value="-622685174" label="enable-pdf-material-ui"/>
+  <int value="-610411643" label="enable-printer-app-search"/>
   <int value="-604814313" label="enable-pinch"/>
   <int value="-601384286" label="disable-contextual-search"/>
   <int value="-579192400" label="disable-input-view"/>