Reland "[ios] Bump minimum Xcode requirement to 10."

This reverts commit 41dde5a43e59031aa725fc0718485beb95a1769a.

Reason for revert: cronet is now on Xcode 10, webrtc is pending

Original change's description:
> Revert "[ios] Bump minimum Xcode requirement to 10."
> 
> This reverts commit ae6e851448baf9edca90656f50f8fa919c1a7b71.
> 
> Reason for revert: cronet and webrtc aren't on Xcode 10 yet.
> 
> Original change's description:
> > [ios] Bump minimum Xcode requirement to 10.
> > 
> > Change-Id: I60521b36cb903ca18d59fe88caa1a9c7b1aa9e87
> > Reviewed-on: https://chromium-review.googlesource.com/1182377
> > Reviewed-by: Sylvain Defresne <[email protected]>
> > Reviewed-by: Rohit Rao <[email protected]>
> > Reviewed-by: Justin Cohen <[email protected]>
> > Commit-Queue: Justin Cohen <[email protected]>
> > Cr-Commit-Position: refs/heads/master@{#587174}
> 
> [email protected],[email protected],[email protected]
> 
> Change-Id: I815f001ec9c75008791dd46cb1ee39f62248a160
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/1194601
> Reviewed-by: Justin Cohen <[email protected]>
> Commit-Queue: Justin Cohen <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#587222}

[email protected],[email protected],[email protected]

Change-Id: Id5f352d6d38b736ceea1347bcb05c00a55799e97
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1196243
Reviewed-by: Justin Cohen <[email protected]>
Commit-Queue: Justin Cohen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#587435}
diff --git a/docs/ios/build_instructions.md b/docs/ios/build_instructions.md
index 8f7a6eb..8d5788b 100644
--- a/docs/ios/build_instructions.md
+++ b/docs/ios/build_instructions.md
@@ -13,7 +13,7 @@
 ## System requirements
 
 * A 64-bit Mac running 10.12.6 or later.
-* [Xcode](https://developer.apple.com/xcode) 9.0+.
+* [Xcode](https://developer.apple.com/xcode) 10.0+.
 * The current version of the JDK (required for the Closure compiler).
 
 ## Install `depot_tools`