[Fuchsia] Update PathProvideFuchsia to avoid blocking IO.

Previously GetPackageRoot() was calling base::FileExists(), which requires
blocking IO, i.e. it DCHECKs on some threads. Updated the logic used to
detect that the app is packaged to avoid blocking IO.

Bug: 796779, 778467
Change-Id: I3ae8f5fefea182295a7fda4ff40964f21ba63e5c
Reviewed-on: https://chromium-review.googlesource.com/876660
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531308}

base/base_paths_fuchsia.cc

@@ -21,7 +21,7 @@ constexpr char kPackageRoot[] = "/pkg";
 
 base::FilePath GetPackageRoot() {
   base::FilePath path_obj(kPackageRoot);
-  if (PathExists(path_obj)) {
+  if (path_obj.IsParent(base::CommandLine::ForCurrentProcess()->GetProgram())) {
     return path_obj;
   } else {
     return base::FilePath();