Fix font fallback Safari 10 for iOS

[dengjeffrey]

What this PR tries to accomplish

• Partially fixes Font families might not really loaded in WebKit based browsers in specific networks conditions #365, for the iOS portion.

  • On Safari 10 for iOS after setting a font, the FontRuler will report a width that is different than any of the fallback FontRuler widths.
  • Once the font has loaded, the width is then changed again to the correct width.

• Extend coverage of Disable native font loading in Safari 10 #352 for iOS devices running Safari 10.

  • navigator.userAgent for iOS devices do not contain *Version\/10 match which fails the check for when to use the native loader

How it accomplishes them

Partially fixes #365, for the iOS portion.

• Add Safari 10 to the fallback bug check.
• Create a FontRuler for a mock font with the same styling and record the width as a lastResortWidth.
• Since the fallback bug is present, the font will be considered loaded when the font width does not match any of the widths in lastResortWidth

Extend coverage of #352 for iOS devices running Safari 10.

• Match on AppleWebKit version to determine if webkit version is equivalent to the one used in Safari 10

[belartodmarinus]

diff gets a bit confused here because you've removed an empty line after this.started_ = goog.now();