Fix nested unicode-bidi: isolate
When we have a nested isolate renderer if the outer isolate has a text and this text is not the first child, the isolated chain is not rendered correctly. This happens because constructBidiRunsForSegment uses always the first inner isolated renderer as isolated inline container. This patch fixes the behavior described changing containingIsolate to find the right isolated container.

BUG=274717

Review URL: https://chromiumcodereview.appspot.com/23298002

git-svn-id: svn://svn.chromium.org/blink/trunk@156261 bbb929c8-8fbe-4397-9dbb-9b2b20218538