Add launcherPage.setEnabled API.

This CL adds a launcherPage.setEnabled API which enables or disables the
click zone in the app list which shows and hides the custom launcher page.

BUG=438948

Review URL: https://codereview.chromium.org/778643009

Cr-Commit-Position: refs/heads/master@{#307864}
diff --git a/extensions/browser/extension_function_histogram_value.h b/extensions/browser/extension_function_histogram_value.h
index 42fd7f0..54f83ee 100644
--- a/extensions/browser/extension_function_histogram_value.h
+++ b/extensions/browser/extension_function_histogram_value.h
@@ -999,6 +999,7 @@
   COPRESENCEENDPOINTS_DESTROYLOCALENDPOINT,
   COPRESENCEENDPOINTS_SEND,
   INLINE_INSTALL_PRIVATE_INSTALL,
+  LAUNCHERPAGE_SETENABLED,
   // Last entry: Add new entries above and ensure to update
   // tools/metrics/histograms/histograms.xml.
   ENUM_BOUNDARY