Roll chromium_revision 1a6cf4da7c..f1e2718a3f (431807:431838)

Use system Xcode on Mac instead of the hermetic toolchain which
is now the default in Chromium. WebRTC needs the 10.12 SDK to
compile successfully, which is not availble in the hermetic
toolchain Chromium is rolling out to Googlers.

Change log: https://chromium.googlesource.com/chromium/src/+log/1a6cf4da7c..f1e2718a3f
Full diff: https://chromium.googlesource.com/chromium/src/+/1a6cf4da7c..f1e2718a3f

No dependencies changed.
No update to Clang.

TBR=ehmaldonado@webrtc.org
BUG=webrtc:6700

Review-Url: https://codereview.webrtc.org/2496113002
Cr-Commit-Position: refs/heads/master@{#15057}
diff --git a/DEPS b/DEPS
index 499bc2e..311a881 100644
--- a/DEPS
+++ b/DEPS
@@ -6,7 +6,7 @@
 vars = {
   'extra_gyp_flag': '-Dextra_gyp_flag=0',
   'chromium_git': 'https://chromium.googlesource.com',
-  'chromium_revision': '1a6cf4da7c27c9b66deb6dcd9dc98458536d952f',
+  'chromium_revision': 'f1e2718a3ff89c80691a50f8ea2503cbb9aa97ee',
 }
 
 # NOTE: Use http rather than https; the latter can cause problems for users
diff --git a/build_overrides/build.gni b/build_overrides/build.gni
index 3d17e87..de26558 100644
--- a/build_overrides/build.gni
+++ b/build_overrides/build.gni
@@ -35,3 +35,7 @@
 # limit, making them requiring symbol_level=2. WebRTC doesn't hit that problem
 # so we just ignore that assert. See https://crbug.com/648948 for more info.
 ignore_elf32_limitations = true
+
+# Use system Xcode installation instead of the Chromium bundled Mac toolchain,
+# since it contains only SDK 10.11, not 10.12 which WebRTC needs.
+use_system_xcode = true