Define default values for primitive-type fields of ...NavigationParams.

This CL provides explicit default values for most of the primitive-type
fields of CommonNavigationParams, BeginNavigationParams and
CommitNavigationParams.  The explicit initialization is mostly a
readability aid, since otherwise the auto-generated default constructor
will just call the default constructor of the fields:
    BeginNavigationParams::BeginNavigationParams()
        : initiator_routing_id(),
          headers(),
          load_flags()
    ...
Still, being explicit in the mojom file helps:
1. It makes it clear that the default has been explicitly selected by a
   human, rather then provided by the compiler and/or other tools.
2. It doesn't require the reader to understand that omitting the
   initializer is safe and will zero-initialize POD values.
3. It does make a difference in cases when non-zero defaults are better
  - for example when initializing initiator_routing_id to
  MSG_ROUTING_NONE.  (rather than falling back to zero-initializatio

Note that default initial values cannot be provided for [Native] enums.

Bug: 1107269
Change-Id: I9a1dfc9467d52ff9485cc124ab1859cf50b6b345
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341892
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796013}

content/common/navigation_params.mojom

@@ -97,33 +97,38 @@ struct BeginNavigationParams {
   // Routing id of the initiator of the navigation. This routing id may no
   // longer be valid by the time the navigation is seen by the browser process,
   // if the initiator frame is deleted before the navigation IPC arrives. A
-  // value of MSG_ROUTING_NONE indicates this navigation is not associated
-  // with a frame.
-  int32 initiator_routing_id;
+  // default value of MSG_ROUTING_NONE indicates this navigation is not
+  // associated with a frame.
+  int32 initiator_routing_id = -2;  // -2 == MSG_ROUTING_NONE
 
   // Additional HTTP request headers.
   string headers;
 
   // net::URLRequest load flags (net::LOAD_NORMAL) by default).
-  int32 load_flags;
+  int32 load_flags = 0;  // 0 == net::LOAD_NORMAL
 
   // True if the ServiceWorker should be skipped.
-  bool skip_service_worker;
+  bool skip_service_worker = false;
 
   // Indicates the request context type.
-  blink.mojom.RequestContextType request_context_type;
+  blink.mojom.RequestContextType request_context_type =
+      blink.mojom.RequestContextType.UNSPECIFIED;
 
   // Indicates the request destination.
-  network.mojom.RequestDestination request_destination;
+  network.mojom.RequestDestination request_destination =
+      network.mojom.RequestDestination.kEmpty;
 
   // The mixed content context type for potential mixed content checks.
+  //
+  // [Native] enums are by default initialized to 0
+  // (i.e. blink::WebMixedContentContextType::kNotMixedContent below).
   MixedContentContextType mixed_content_context_type;
 
   // Whether or not the navigation has been initiated by a form submission.
-  bool is_form_submission;
+  bool is_form_submission = false;
 
   // Whether or not the navigation has been initiated by a link click.
-  bool was_initiated_by_link_click;
+  bool was_initiated_by_link_click = false;
 
   // See WebSearchableFormData for a description of these.
   url.mojom.Url searchable_form_url;
@@ -181,6 +186,8 @@ struct CommonNavigationParams {
   blink.mojom.Referrer referrer;
 
   // The type of transition.
+  //
+  // [Native] enums are by default initialized to 0 (i.e. PAGE_TRANSITION_LINK).
   PageTransition transition;
 
   // Type of navigation.
@@ -206,7 +213,7 @@ struct CommonNavigationParams {
   // Bitmask that has whether or not to request a Preview version of the
   // document for various preview types or let the browser decide.
   // Defined in third_party/blink/public/common/loader/previews_state.h.
-  int32 previews_state;
+  int32 previews_state = 0;  // 0 == PREVIEWS_UNSPECIFIED
 
   // The navigationStart time exposed through the Navigation Timing API to JS.
   mojo_base.mojom.TimeTicks navigation_start;