Fix display of 404s in SkyShell to show error text.

Fix display of 404s in SkyShell to show error text.

In MojoShell we never hit this path since the NetworkFetcher
aborts earlier on, not knowing what content_handler to use.

R=ojan@chromium.org, ianh@google.com

Committed: https://chromium.googlesource.com/external/mojo/+/43e85d0ffbf8a7f714dd43afd65c022dfa7cbc25

[ojan, 2015-04-17 02:21:45]

ojan@chromium.org changed reviewers:
+ ojan@chromium.org

[ojan, 2015-04-17 02:21:45]

lgtm

[Hixie, 2015-04-17 16:51:11]

This suffers from cross-format injection. If you want to do it by constructing a
string, you need to escape the data you're inserting. Note that medium term if
we're removing the parser as a built-in feature this will have to change to
building the DOM manually anyway (albeit maybe it'll be a different kind of DOM
than we have now).

[eseidel, 2015-04-21 22:38:31]

Committed patchset #1 (id:1) manually as
43e85d0ffbf8a7f714dd43afd65c022dfa7cbc25 (presubmit successful).