Increase minimum iOS version requirements to iOS 9.0.

Chrome on iOS requires iOS 9.0 as it uses WKWebView (and the version
from iOS 8.0 lacks some required features). Increate the minimum iOS
version in build/common.gypi.

BUG=569158

Review URL: https://codereview.chromium.org/1704933002

Cr-Original-Commit-Position: refs/heads/master@{#375862}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 77193364a92ca0636fe8774ce6e37bedff9bbf88

common.gypi

@@ -1690,7 +1690,7 @@
         # not using the "current" SDK.
         'ios_sdk%': '',
         'ios_sdk_path%': '',
-        'ios_deployment_target%': '7.0',
+        'ios_deployment_target%': '9.0',
 
         'conditions': [
           # ios_product_name is set to the name of the .app bundle as it should

config/ios/ios_sdk.gni

@@ -10,7 +10,7 @@ declare_args() {
   use_ios_simulator = target_cpu == "x86" || target_cpu == "x64"
 
   # Version of iOS that we're targeting.
-  ios_deployment_target = "7.0"
+  ios_deployment_target = "9.0"
 
   # The iOS Code signing identity to use
   # TODO(GYP), TODO(sdfresne): Consider having a separate