PDFiumEngine::~PDFiumEngine - Unload all pages_ before destroying any of them.

PDFiumEngine::~PDFiumEngine - Unload() all pages_ before destroying any of them.

Currently, destroying a PDFiumPage triggers an Unload() which can result
in an access to other PDFiumPages. But these other pages may already have
been destroyed as a previous element via STLDeleteElements(&pages_).

Instead, unload all pages first, then destroy them all afterwards so that
no access to other pages will be attempted after the destruction of the
first one has begun.

Move the STLDeleteElements() call to the bottom, to make it more closely
mimic the ordering that might occur should pages_ someday be switched
from vector to ScopedPtrVector or similar.

BUG=384365
Committed: https://crrev.com/885a59eed02ac54886360c2e9b5d2d1a0e42719b
Cr-Commit-Position: refs/heads/master@{#293800}

[Tom Sepez, 2014-09-04 22:31:52]

tsepez@chromium.org changed reviewers:
+ gene@chromium.org

[Tom Sepez, 2014-09-04 22:35:12]

Gene, please review.

[gene, 2014-09-06 00:19:31]

lgtm!

[Tom Sepez, 2014-09-08 17:40:21]

The CQ bit was checked by tsepez@chromium.org

[commit-bot: I haz the power, 2014-09-08 17:41:38]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tsepez@chromium.org/544873002/1

[commit-bot: I haz the power, 2014-09-08 22:44:00]

Committed patchset #1 (id:1) as 1013115488a3e0a7f61e8d1bc9222d5a5d281862

[commit-bot: I haz the power, 2014-09-10 03:48:36]

Patchset 1 (id:??) landed as
https://crrev.com/885a59eed02ac54886360c2e9b5d2d1a0e42719b
Cr-Commit-Position: refs/heads/master@{#293800}