Extensions: Clarify ManifestHandler interface expectations.

extensions/common/manifest_handler.h

Index: extensions/common/manifest_handler.h
diff --git a/extensions/common/manifest_handler.h b/extensions/common/manifest_handler.h
index d1cf4648925149d133d86cb8878edfb417364708..f37729129f27a9135c0b3f3a64576a9bbd5e0291 100644
--- a/extensions/common/manifest_handler.h
+++ b/extensions/common/manifest_handler.h
@@ -29,11 +29,13 @@ class ManifestHandler {
   // Attempts to parse the extension's manifest.
   // Returns true on success or false on failure; if false, |error| will
   // be set to a failure message.
+  // This does not perform any IO operations.

[lazyboy, 2017/04/26 22:53:03]

Thinking about it, this doesn't seem that helpful, b/c: 1) by default we assume
we're not doing IO if not explicitly specified. 2) we're not ensuring the fact
in code (opposed to ValidateExtension, where we assert it).
You can remove this but feel free to keep it if you feel necessary.

[karandeepb, 2017/04/26 22:57:38]

We were doing IO for Validate as in the example I pointed out.

. 2) we're not ensuring the fact
I think this is helpful for those implementing a ManifestHandler. That's why I
wanted it to be a guidance -> "This should not perform any IO operations". 

   virtual bool Parse(Extension* extension, base::string16* error) = 0;
 
   // Validate that files associated with this manifest key exist.
   // Validation takes place after parsing. May also append a series of
   // warning messages to |warnings|.
+  // This may perform IO operations.
   //
   // Otherwise, returns false, and a description of the error is
   // returned in |error|.
@@ -89,7 +91,8 @@ class ManifestHandler {
   // this extension.
   static bool ParseExtension(Extension* extension, base::string16* error);
 
-  // Call Validate on all registered manifest handlers for this extension.
+  // Call Validate on all registered manifest handlers for this extension. This
+  // may perform IO operations.
   static bool ValidateExtension(const Extension* extension,
                                 std::string* error,
                                 std::vector<InstallWarning>* warnings);