Return error on JSON request failures

Return error on JSON request failures

Previously on a TracingController error, OnBeginRequest returns false,
and then WebUIDataSource will return the default resource (tracing.html).
This will cause a successful XHR request and a JSON parse failure in
tracing frontend code. Return "##ERROR##" to express the error so that
the frontend can handle the error.

BUG=331286
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=243162

[dsinclair, 2014-01-06 15:20:33]

https://codereview.chromium.org/117323008/diff/1/content/browser/tracing/trac...
File content/browser/tracing/tracing_ui.cc (right):

https://codereview.chromium.org/117323008/diff/1/content/browser/tracing/trac...
content/browser/tracing/tracing_ui.cc:150: std::string error("##ERROR##");
Why not make this valid json? {"error": "invalid path"}?

[Xianzhu, 2014-01-06 17:11:26]

https://codereview.chromium.org/117323008/diff/1/content/browser/tracing/trac...
File content/browser/tracing/tracing_ui.cc (right):

https://codereview.chromium.org/117323008/diff/1/content/browser/tracing/trac...
content/browser/tracing/tracing_ui.cc:150: std::string error("##ERROR##");
On 2014/01/06 15:20:33, dsinclair wrote:
> Why not make this valid json? {"error": "invalid path"}?

I didn't think of this, just randomly selected the error indicator. However in
JS UI code (https://codereview.appspot.com/47660043/), error condition is
checked in XHR handling before JSON decoding, so using a plain string seems more
convenient.

[dsinclair, 2014-01-06 17:36:35]

lgtm

[commit-bot: I haz the power, 2014-01-06 17:41:26]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/wangxianzhu@chromium.org/117323008/1

[commit-bot: I haz the power, 2014-01-06 20:55:08]

Change committed as 243162