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When a GURL's path is canonicalized, the code is trying to handle
nested escape sequences. It keeps the index of the last seemingly
invalid "%" in a variable called last_invalid_percent_index, so that
ssequences like "%%41B" can be transformed to "%AB".

However, it can happen, that an already output part of the path is
invalidated and cut off, such as in "abc/../", which is equivalent to
"".

The bug in the current code is that last_invalid_percent_index is not
reset if the part of the output which it points to gets invalidated.
This resulted in the mysterious inserting of the hex code of "%" in
the middle of the escape sequence of ">" when canonicalizing the path
"5%/../>%41".

This CL simply makes sure that last_invalid_percent_index is reset
whenever it points beyond the end of the current output.

Bug: 1080890
Change-Id: I04a5246ecbb66b645d796ec88babd74383d18661
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587060
Auto-Submit: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837031}