Avoid OOB memcpy in chrome_pdf::CopyImage.

      This is a re-work of palmer's patch at https://codereview.chromium.org/515023002/ which has more context, but comes down to stricter bounds checking.

We also correct an arithmetic bug when copying the image behind a control that is positioned before the origin of the image.

BUG=398384

Review URL: https://codereview.chromium.org/519873002

Cr-Commit-Position: refs/heads/master@{#293213}