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-Commit-Position: refs/heads/master@{#375862}