Fix a crash when saving a <canvas> or <img> image which is large.

The problem is caused by size-limits of GURL. (the limit < 2MiB)
We can use string of data url instead of GURL to avoid the size-limits.

For this end, added a new method to Blink side:
  https://crrev.com/518043003

BUG=69227, 401888

Review URL: https://codereview.chromium.org/518693002

Cr-Commit-Position: refs/heads/master@{#295405}
11 files changed