[ios] Fix incorrect use of rebase_path() in ios/web_view

The parameters of a script invocation needs to be relative to the
directory where the script is invoked which is available as the
`root_build_dir` variable.

Using `root_out_dir` is incorrect as it has a different value for
secondary toolchains (i.e. current_toolchain != default_toolchain).

Bug: none
Change-Id: I3fc329e27657377493c3c7cca6b7ff060531ec87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2489624
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819371}

ios/web_view/BUILD.gn

@@ -634,8 +634,8 @@ if (additional_toolchains == [] || current_toolchain == default_toolchain) {
 
       outputs = [ _output ]
 
-      args = [ rebase_path(_output, root_out_dir) ] +
-             rebase_path(_dsyms, root_out_dir)
+      args = [ rebase_path(_output, root_build_dir) ] +
+             rebase_path(_dsyms, root_build_dir)
 
       deps = [ ":web_view" ]
     }