Fix resolving protocol-agnostic <svg:image> URLs in about:blank iframes

Fix resolving protocol-agnostic <svg:image> URLs in about:blank iframes

When resolving a protocol-agnostic URL against a base of 'about:blank',
the result would be an invalid (and empty) URL, because of missing special-
casing of 'about:blank' - which is present in
Document::completeURLWithOverride.
Fix by modifying SVGImageLoader::sourceURI to always resolve the URL using
Document::completeURLWithOverride to not miss out on this special-casing
logic for 'about:blank' base URLs.

BUG=379100
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176043

[fs, 2014-06-12 16:18:01]

(Couldn't quite shake the "crazy!" feeling here, but...)

[pdr., 2014-06-12 16:24:28]

On 2014/06/12 16:18:01, fs wrote:
> (Couldn't quite shake the "crazy!" feeling here, but...)

LGTM

[pdr., 2014-06-12 16:24:31]

The CQ bit was checked by pdr@chromium.org

[commit-bot: I haz the power, 2014-06-12 16:25:37]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/fs@opera.com/329943003/1

[commit-bot: I haz the power, 2014-06-12 17:33:27]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/11771)

[commit-bot: I haz the power, 2014-06-12 18:39:21]

Change committed as 176043