Bug 1530642 - Added more doc on ProcType and GetProcInfo - r=jesup

Added more docs for type synchronization, and about GetProcInfo

Differential Revision: https://phabricator.services.mozilla.com/D21205

--HG--
extra : moz-landing-system : lando

dom/ipc/ContentParent.h

@@ -42,7 +42,7 @@
 #define CHILD_PROCESS_SHUTDOWN_MESSAGE \
   NS_LITERAL_STRING("child-process-shutdown")
 
-// These must match the similar ones in E10SUtils.jsm.
+// These must match the similar ones in E10SUtils.jsm and ProcInfo.h.
 // Process names as reported by about:memory are defined in
 // ContentChild:RecvRemoteType.  Add your value there too or it will be called
 // "Web Content".

toolkit/modules/E10SUtils.jsm

@@ -42,7 +42,7 @@ function getAboutModule(aURL) {
 
 const NOT_REMOTE = null;
 
-// These must match any similar ones in ContentParent.h.
+// These must match any similar ones in ContentParent.h and ProcInfo.h
 const WEB_REMOTE_TYPE = "web";
 const FILE_REMOTE_TYPE = "file";
 const EXTENSION_REMOTE_TYPE = "extension";

widget/ProcInfo.h

@@ -66,6 +66,12 @@ struct ProcInfo {
 
 typedef MozPromise<ProcInfo, nsresult, true> ProcInfoPromise;
 
+/*
+ * GetProcInfo() uses a background thread to perform system calls.
+ *
+ * Depending on the platform, this call can be quite expensive and the
+ * promise may return after several ms.
+ */
 RefPtr<ProcInfoPromise> GetProcInfo(base::ProcessId pid, int32_t childId,
                                     const ProcType& type);