[ios] Fixed compile error in web_view license

There is no path between 3p/perfetto and ios/web_view

TBR=ichikawa@google.com

Bug: 1065322
Change-Id: I72c14b7a80d3086cfb1e449e571e094e50bcd644
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2124368
Commit-Queue: David Jean <djean@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753961}

ios/web_view/BUILD.gn

@@ -564,6 +564,8 @@ if (additional_toolchains == [] || current_toolchain == default_toolchain) {
       "//ios/web_view",
       "--gn-out-dir",
       ".",
+      "--target-os",
+      "ios",
     ]
   }