[ios] Remove duplicate code in "runtime_library" config definition

The code to rebase the path to the sdk was duplicated between clang
and swift. Use a local variable initialised with the value that was
computed in the two location, then use it to configure "-isysroot"
for clang and "-sdk" for swiftc.

Bug: none
Change-Id: Ic3794ceeea84c67404763cf5d86fc32c201e3a4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2471357
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817444}

build/config/ios/BUILD.gn

@@ -84,26 +84,20 @@ config("compiler") {
 # that is iOS-only. Please see that target for advice on what should go in
 # :runtime_library vs. :compiler.
 config("runtime_library") {
+  _sysroot_path = sysroot
   if (ios_enable_relative_sdk_path) {
-    common_flags = [
-      "-isysroot",
-      rebase_path(sysroot, root_build_dir),
-    ]
-    swiftflags = [
-      "-sdk",
-      rebase_path(sysroot, root_build_dir),
-    ]
-  } else {
-    common_flags = [
-      "-isysroot",
-      sysroot,
-    ]
-    swiftflags = [
-      "-sdk",
-      sysroot,
-    ]
+    _sysroot_path = rebase_path(sysroot, root_build_dir)
   }
 
+  common_flags = [
+    "-isysroot",
+    _sysroot_path,
+  ]
+  swiftflags = [
+    "-sdk",
+    _sysroot_path,
+  ]
+
   if (use_xcode_clang && enable_ios_bitcode && !use_ios_simulator) {
     if (is_debug) {
       common_flags += [ "-fembed-bitcode-marker" ]