[gn] Fix config for icu data file and swarming

Fix after:
https://codereview.chromium.org/2042253002/
https://codereview.chromium.org/2049743002/

Using the data file is the default now and it shouldn't
be overridden in v8's build_overrides. Also the isolate
logic now needs to include the gni file specifying the
flag.

BUG=chromium:616033
TBR=jochen@chromium.org, vogelheim@chromium.org
NOTRY=true

Review-Url: https://codereview.chromium.org/2062363002
Cr-Commit-Position: refs/heads/master@{#36998}

build_overrides/v8.gni

@@ -18,7 +18,6 @@ if (((v8_target_arch == "ia32" ||
   v8_enable_gdbjit = true
 }
 
-icu_use_data_file = false
 v8_imminent_deprecation_warnings = true
 
 # Add simple extras solely for the purpose of the cctests.

gni/isolate.gni

@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/sanitizers/sanitizers.gni")
+import("//third_party/icu/config.gni")
 import("v8.gni")
 
 declare_args() {