Don't send empty strings for nonexistent GURLs.

addLinkRange expects non-nil URL pointers to point to nonempty URLs.  But
infobars may have empty link URLs, for example if they're going to custom-handle
the link click by doing something other than a simple navigation.  So pass nil
in those cases.

This fixes a DCHECK exposed by adding infobar UI tests (yay!).

BUG=686285
TEST=none

Change-Id: I59235a12ea85cc300725e36fc815b07f845ec23e
Reviewed-on: https://chromium-review.googlesource.com/849418
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526899}

chrome/browser/ui/cocoa/infobars/confirm_infobar_controller.mm

@@ -127,8 +127,11 @@
   if (linkLength != 0) {
     NSColor* linkColor =
         skia::SkColorToCalibratedNSColor(chrome_style::GetLinkColor());
+    GURL linkUrl = delegate->GetLinkURL();
+    NSString* urlString =
+        linkUrl.is_valid() ? base::SysUTF8ToNSString(linkUrl.spec()) : nil;
     [view addLinkRange:NSMakeRange(linkOffset, linkLength)
-               withURL:base::SysUTF8ToNSString(delegate->GetLinkURL().spec())
+               withURL:urlString
              linkColor:linkColor];
   }
 }