GN: fix a DCHECK() when generating Visual Studio solution files

VisualStudioWriter::WriteSolutionFileContents() used RebasePath()
with |source_root| = "/" which is forbidden. It always uses system
absolute paths, thus we can use |source_root| = "".

R=brettw@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1723613003

Cr-Original-Commit-Position: refs/heads/master@{#377258}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: bbe9c04f0889293cea85b6806145e26e1670a223

tools/gn/visual_studio_writer.cc

diff --git a/tools/gn/visual_studio_writer.cc b/tools/gn/visual_studio_writer.cc
index 7748fbe..4b64f1e 100644
--- a/tools/gn/visual_studio_writer.cc
+++ b/tools/gn/visual_studio_writer.cc
@@ -606,14 +606,14 @@
   SourceDir solution_dir(FilePathToUTF8(solution_dir_path));
   for (const SolutionEntry* folder : folders_) {
     out << "Project(\"" << kGuidTypeFolder << "\") = \"(" << folder->name
-        << ")\", \"" << RebasePath(folder->path, solution_dir, "/") << "\", \""
+        << ")\", \"" << RebasePath(folder->path, solution_dir) << "\", \""
         << folder->guid << "\"" << std::endl;
     out << "EndProject" << std::endl;
   }
 
   for (const SolutionEntry* project : projects_) {
     out << "Project(\"" << kGuidTypeProject << "\") = \"" << project->name
-        << "\", \"" << RebasePath(project->path, solution_dir, "/") << "\", \""
+        << "\", \"" << RebasePath(project->path, solution_dir) << "\", \""
         << project->guid << "\"" << std::endl;
     out << "EndProject" << std::endl;
   }