Create /var/cache/shared_extensions for installed shared extensions

This directory needs to be writable by Chrome process because
Chrome will manage content of the directory. Integrity checking
will be done by Chrome using iextensions::ContentVerifier. Installation
sharing is off default now (controlled by Chrome flag that will not be
exposed in about:flags until feature fully implemented).

BUG=chromium:235263
TEST=manual
CQ-DEPEND=CL:200628

Change-Id: Ifb58e66a7bdb35c460cd3ef41246e3f44ba3beb7
Reviewed-on: https://chromium-review.googlesource.com/200615
Reviewed-by: Dmitry Polukhin <dpolukhin@chromium.org>
Commit-Queue: Dmitry Polukhin <dpolukhin@chromium.org>
Tested-by: Dmitry Polukhin <dpolukhin@chromium.org>
diff --git a/init/ui.conf b/init/ui.conf
index 72b98f1..b2dae97 100644
--- a/init/ui.conf
+++ b/init/ui.conf
@@ -90,6 +90,12 @@
   mkdir -m 0700 -p /var/cache/device_local_account_extensions
   chown chronos:chronos /var/cache/device_local_account_extensions
 
+  # Create the directory for shared installed extensions.
+  # Shared extensions are validated at runtime by the browser.
+  # These extensions are read and written by chronos.
+  mkdir -m 0700 -p /var/cache/shared_extensions
+  chown chronos:chronos /var/cache/shared_extensions
+
   # Enable us to keep track of the user's chosen TZ.
   # Default to Pacific timezone if we don't have one set
   TIMEZONE_DIR=/var/lib/timezone