Move site_engagement.mojom to chrome/browser/engagement.

To allow use of the same SiteEngagementInfo structure by both the site
engagement UI and the important sites implementation, this CL moves the
Mojo definitions out of c/b/ui/webui/engagement.

BUG=703848
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2783433003
Cr-Commit-Position: refs/heads/master@{#460248}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 6bee5f7a..770f1f2 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -4258,7 +4258,7 @@
     # Depend only on the generated mojo bindings since we read the .mojom.js
     # file, rather than the whole mojo target which will link the C++ bindings.
     "//chrome/app:chrome_content_manifest_overlays",
-    "//chrome/browser/ui/webui/engagement:mojo_bindings__generator",
+    "//chrome/browser/engagement:mojo_bindings__generator",
     "//chrome/browser/ui/webui/omnibox:mojo_bindings__generator",
     "//chrome/browser/ui/webui/usb_internals:mojo_bindings__generator",
     "//device/bluetooth/public/interfaces:experimental_interfaces__generator",