Updating Simulator instructions for Xcode9 and minimum OS version.

iossim no longer automatically launches the Simulator, that must be
done manually. Also, macOS 10.12 is required for Xcode 9.

TBR=rohitrao

Bug: none.
Change-Id: Icab37c48a6b79fbbe3e5a99b89a67058e215ca98
Reviewed-on: https://chromium-review.googlesource.com/824562
Commit-Queue: Mike Pinkerton <pinkerton@chromium.org>
Reviewed-by: Mike Pinkerton <pinkerton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523786}

docs/ios/build_instructions.md

@@ -12,7 +12,7 @@ Are you a Google employee? See
 
 ## System requirements
 
-* A 64-bit Mac running 10.11+.
+* A 64-bit Mac running 10.12.6 or later.
 * [Xcode](https://developer.apple.com/xcode) 9.0+.
 * The current version of the JDK (required for the Closure compiler).
 
@@ -226,6 +226,10 @@ command line, you can use `iossim`. For example, to run a debug build of
 $ out/Debug-iphonesimulator/iossim out/Debug-iphonesimulator/Chromium.app
 ```
 
+With Xcode 9, `iossim` no longer automatically launches the Simulator. This must now
+be done manually from within Xcode (`Xcode > Open Developer Tool > Simulator`), and
+also must be done *after* running `iossim`.
+
 ### Passing arguments
 
 Arguments needed to be passed to the test application through `iossim`, such as