Prerender: Allow prerendering pages to create blob URL

Previously, prerendering pages cannot create blob URLs, as this API
uses a sync IPC to communicate, and we would cancel prerendering by
default in this case to ensure prerendering is safe.
After auditing it (see reasoning), we think creating an object URL seems
safe, and we can grant it during prerendering.

Reasoning:
- The blob url should be same-origin as the prerendering origin, so we
  do not touch other origins.
- The security concerns should be addressed by this API.
  https://w3c.github.io/FileAPI/#security-discussion


Bug: 1406125
Change-Id: I3c94e45fe59adb7227a71a702a4793ee8281b395
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4583910
Commit-Queue: Lingqi Chi <lingqi@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1152423}