Headless: convert --user-data-dir to absolute when passed a relative path

Bug: 1090988
Change-Id: I59d49a006d61c948a507633cbb4ed965ec2d11b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2447092Reviewed-by: Peter Kvitek <kvitekp@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813452}

headless/lib/browser/headless_browser_context_impl.cc

@@ -151,10 +151,15 @@ void HeadlessBrowserContextImpl::Close() {
 
 void HeadlessBrowserContextImpl::InitWhileIOAllowed() {
   if (!context_options_->user_data_dir().empty()) {
-    path_ = context_options_->user_data_dir().Append(kDefaultProfileName);
+    base::FilePath path =
+        context_options_->user_data_dir().Append(kDefaultProfileName);
+    if (!path.IsAbsolute())
+      path = base::PathService::CheckedGet(base::DIR_CURRENT).Append(path);
+    path_ = std::move(path);
   } else {
     base::PathService::Get(base::DIR_EXE, &path_);
   }
+  DCHECK(path_.IsAbsolute());
 }
 
 std::unique_ptr<content::ZoomLevelDelegate>