Update requirements for building for the Mac

Chromium requires the 10.14 SDK. As per
<https://en.wikipedia.org/wiki/Xcode>, that version of the
SDK came with Xcode 10.0, which requires at least macOS
10.13.6.

Update the documentation accordingly.

Bug: none
Change-Id: I9ade5aedd6811ba6a4f4c1951191f21af0bd58fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013634
Auto-Submit: Avi Drissman <avi@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733979}

docs/mac_build_instructions.md

@@ -12,16 +12,16 @@ Are you a Google employee? See
 
 ## System requirements
 
-*   A 64-bit Mac running 10.12+.
-*   [Xcode](https://developer.apple.com/xcode) 8+
-*   The OS X 10.12 SDK. Run
+*   A 64-bit Mac running 10.13+.
+*   [Xcode](https://developer.apple.com/xcode) 10+
+*   The OS X 10.14 SDK. Run
 
     ```shell
     $ ls `xcode-select -p`/Platforms/MacOSX.platform/Developer/SDKs
     ```
 
     to check whether you have it.  Building with a newer SDK works too, but
-    the releases currently use the 10.12 SDK.
+    the releases currently use the 10.14 SDK.
 
 ## Install `depot_tools`