Add Tracing.getCategories to protocol.json.

Source/devtools/protocol.json

Index: Source/devtools/protocol.json
diff --git a/Source/devtools/protocol.json b/Source/devtools/protocol.json
index efb1b5ad5f256922ea642233e8eccae62efe40ec..c261a707a6012f3d229ccddb7129e649fde4895a 100644
--- a/Source/devtools/protocol.json
+++ b/Source/devtools/protocol.json
@@ -4126,6 +4126,14 @@
                 "name": "end",
                 "description": "Stop trace events collection.",
                 "handlers": ["browser", "frontend"]
+            },
+            {
+                "name": "getCategories",
+                "description": "Gets supported tracing categories.",
+                "returns": [
+                    { "name": "categories", "type": "array", "description": "A list of supported tracing categories." }

[caseq, 2014/06/17 18:38:23]

You probably want items: { "type": "string" } here.

[chrishenry, 2014/06/17 18:53:41]

Curious, why should this be string instead of an array? The categories are
really a list of strings. I can of course send them as comma-separated string.

Here's the implementation:
https://codereview.chromium.org/339743004/

This isn't working yet, so far I'm getting undefined from ChromeShell. s:

+                ],
+                "handlers": ["browser", "frontend"]

[chrishenry, 2014/06/17 17:23:50]

I'm not sure about this handlers field. Do you know what this is for? What would
be the right value here? I just copied it from Tracing.end above.

[caseq, 2014/06/17 18:38:23]

This one is correct (this tells the code generator in what components the code
needs to be generated; here, the command will be sent from front-end to the
browser and will not be handled in the renderer, so you just want these two).

[chrishenry, 2014/06/17 18:53:41]

Thank you!

I'm curious why Tracing.start needs to be handled in the renderer, but
Tracing.end doesn't. What does it mean by handled by the renderer?

             }
         ],
         "events": [