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Invalid share targets will now be ignored, rather than showing up in the
picker and being rejected at launch time. This brings the implementation
in line with the spec: https://wicg.github.io/web-share-target/

This CL:
- Makes manifest_parser.cc check the share target url_template placeholder
  syntax.
- Refactors ReplacePlaceholders() so that it does no string copying when
 called with url_template_filled==nullptr

BUG=694380, 788191

Change-Id: Icd2a5af5e3100fd93a3fa3cd9ba00a23905355b7
Reviewed-on: https://chromium-review.googlesource.com/927581
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Matt Giuca <mgiuca@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543763}