[ios] Fixes cutoff Disclosure header subtitle label

Changes the vertical stackView distribution to
UIStackViewDistributionFillProportionally,so the stackView
doesn't cut the subtitle label for lack of space.


Bug: 862744
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I524dacb378515088faae9e99c0db408ad3f9b4b0
Reviewed-on: https://chromium-review.googlesource.com/1147541Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577401}

ios/chrome/browser/ui/table_view/cells/table_view_disclosure_header_footer_item.mm

@@ -110,6 +110,7 @@ constexpr float kRotationNinetyCCW = -(90 / 180.0) * M_PI;
     UIStackView* verticalStack = [[UIStackView alloc]
         initWithArrangedSubviews:@[ _titleLabel, _subtitleLabel ]];
     verticalStack.axis = UILayoutConstraintAxisVertical;
+    verticalStack.distribution = UIStackViewDistributionFillProportionally;
 
     // Disclosure ImageView. Initial pointing direction is to the right.
     _disclosureImageView = [[UIImageView alloc]