The validation of utf8 strings in WebAssembly modules used the character
kBadChar = 0xFFFD to indicate a validation error. However, this
character can appear in a valid utf8 string. This CL fixes this problem
by duplicating some of the code in {Utf8::CalculateValue} and inlining
it directly into Utf8::Validate. Note that Utf8::Validate is used only
for WebAssembly.

Tests for this change are in the WebAssembly spec tests, which I will
update in a separate CL.

R=vogelheim@chromium.org

Change-Id: I8697b9299f3e98a8eafdf193bff8bdff90efd7dc
Reviewed-on: https://chromium-review.googlesource.com/509534Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45476}