Create a new component "handoff" to be shared between Mac and iOS.

The files handoff_utility.{h,mm} were moved from chrome/browser/mac/ to
components/handoff/, but were not otherwise changed.

In an attempt to reduce duplicated conditionals in build files, I introduced
the platform conditionals into the gyp/GN files for the handoff component, and
then included the component on all platforms.

BUG=380419

Review URL: https://codereview.chromium.org/709123003

Cr-Commit-Position: refs/heads/master@{#304257}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 4d0c425..ed96e01 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -81,6 +81,7 @@
     "//components/feedback",
     "//components/gcm_driver",
     "//components/google/core/browser",
+    "//components/handoff",
     "//components/history/core/browser",
     "//components/history/core/common",
     "//components/infobars/core",
@@ -249,10 +250,6 @@
       #"browser_app_shim"  TODO(GYP)
     ]
   }
-  if (is_mac || is_ios) {
-    sources += rebase_path(gypi_values.chrome_browser_mac_ios_sources,
-                           ".", "//chrome")
-  }
   if (enable_extensions) {
     public_deps += [ "//chrome/browser/extensions" ]
     deps += [