Bug 1727367 - Part 1: Add prefs for max session store form entry size. r=kashav

Add prefs:
* browser.sessionstore.dom_form_limit
  Limit for how long a value from a form can be to be included in
  session store collection.

* browser.sessionstore.dom_form_max_limit
  Limit how much total data we collect from forms from one site, as in
  the sum of length of values plus sum of length of xpath/id.

Differential Revision: https://phabricator.services.mozilla.com/D125195

modules/libpref/init/StaticPrefList.yaml

@@ -1268,6 +1268,18 @@
   value: 2048
   mirror: always
 
+# Maximum number of characters of form field data per field we collect.
+- name: browser.sessionstore.dom_form_limit
+  type: uint32_t
+  value: 1024*1024*2
+  mirror: always
+
+# Maximum number of characters of form data we collect per origin.
+- name: browser.sessionstore.dom_form_max_limit
+  type: uint32_t
+  value: 1024*1024*50
+  mirror: always
+
 # Minimal interval between two save operations in milliseconds (while the user is active).
 - name: browser.sessionstore.interval
   type: RelaxedAtomicUint32