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The spec says that "When the user agent uses this handler, it must replace the first occurrence of
the exact literal string "%s" in the URL argument".

- Spec: https://html.spec.whatwg.org/multipage/system-state.html#custom-handlers

So, only the first %s can be changed to the given URL.

For example, let's assume this scenario,
  1. navigator.registerProtocolHandler("test", 'http://example.com/%s/url=%s', 'title');"
  2. <a href="test:duplicated_placeholders">this</a>

According to the specification, destination URL should be
  "http://example.com/test%3Aduplicated_placeholders/url=%s".

But, current Chrome implementation replaces all placeholders with the given custom url as below,
  "http://example.com/test%3Aduplicated_placeholders/url=test%3Aduplicated_placeholders"

Firefox also substitutes only first placeholder when url contains multiple placeholders.

Bug: 791912
Change-Id: Ic3d439c68ac35d776afa6b6907ecdc0da774b08e
Reviewed-on: https://chromium-review.googlesource.com/808086
Commit-Queue: Gyuyoung Kim <gyuyoung.kim@lge.com>
Reviewed-by: Matt Giuca <mgiuca@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522360}