[Extensions] Don't show the pending URL for chrome.tabs API navigations

For the pdf extension, treat navigations as renderer-initiated. Ideally,
we want this to become default for all extensions, but that risks
breakage.

Based on creis@'s patch at https://codereview.chromium.org/2475033002/.

BUG=660498
TEST=See bug for repro steps.

Review-Url: https://codereview.chromium.org/2492863003
Cr-Commit-Position: refs/heads/master@{#431726}
4 files changed