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This change is the IOS counterpart to
https://codereview.chromium.org/1101993003/

Code that is used for generating HTML and JavaScript have been moved
to a superclass; this changes the IOS code to use that.

BUG=472797

Review URL: https://codereview.chromium.org/1071413003

Cr-Commit-Position: refs/heads/master@{#330742}