Change TextRun's length() and charactersLength() to return an unsigned

TextRun's m_len and m_charactersLength are unsigned but the accessors
for these values return ints. The values for length and charactersLength
cannot be negative so returning an integer just adds unnecessary casts
and potentially dangerous code.

Review-Url: https://codereview.chromium.org/2018253002
Cr-Commit-Position: refs/heads/master@{#396776}
8 files changed