Add //content:browser dependency on //ppapi:host

content::BrowserPpapiHostImpl references ppapi::host::PpapiHost but dependency in missing in gn.

BUG=

Review URL: https://codereview.chromium.org/1173583002

Cr-Commit-Position: refs/heads/master@{#333699}

content/browser/BUILD.gn

@@ -266,6 +266,7 @@ source_set("browser") {
                            ".",
                            "//content")
     deps += [
+      "//ppapi/host",
       "//ppapi/proxy:ipc",
       "//ppapi/shared_impl",
     ]