Reland #1: Switch the default build configuration to use the 10.10 SDK.

All known problems have been fixed since the first attempt to land this CL.

Original issue's description:
> mac: Switch the default build configuration to use the 10.10 SDK.
>
> This change is intended to be temporary. The goal is to release a Canary
> compiled against the 10.10 SDK, and then to revert this CL.
>
> BUG=463170
>
> Committed: https://crrev.com/42ba22e0b8e87f758804537f64db23ff6438e451
> Cr-Commit-Position: refs/heads/master@{#340992}

TBR=thakis@chromium.org,amineer@chromium.org
BUG=463170

Review URL: https://codereview.chromium.org/1257643005

Cr-Original-Commit-Position: refs/heads/master@{#341261}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 345c0cf47947f209e628941b38d76dca90aee49d

common.gypi

@@ -1901,7 +1901,7 @@
             ['OS=="ios"', {
               'mac_sdk_min%': '10.8',
             }, {  # else OS!="ios"
-              'mac_sdk_min%': '10.6',
+              'mac_sdk_min%': '10.10',
             }],
           ],
           'mac_sdk_path%': '',

config/mac/mac_sdk.gni

@@ -6,7 +6,7 @@ import("//build/config/chrome_build.gni")
 
 declare_args() {
   # Minimum supported version of the Mac SDK.
-  mac_sdk_min = "10.6"
+  mac_sdk_min = "10.10"
 
   # Path to a specific version of the Mac SDKJ, not including a backslash at
   # the end. If empty, the path to the lowest version greater than or equal to