Revert of Remove pageview from map immediately (patchset #1 id:1 of https://codereview.chromium.org/2179283005/ )

Reason for revert:
Looks like this broke linux_asan embedder tests.

https://build.chromium.org/p/client.pdfium/builders/linux_asan/builds/1152/steps/embeddertests/logs/stdio

Original issue's description:
> Remove pageview from map immediately
>
> There seems to be an ownership issue in the page annotation code where removing
> the annotations can result in removing the parent page view. This is fine except
> that removing the parent page view removes the annotations and you can end up
> with a use-after-free.
>
> This CL removes the page view from the documents page map immediately and then
> proceeds with the cleanup. Then, if we try to remove that page again it won't
> be found and we won't double free.
>
> BUG=chromium:630654
>
> Committed: https://pdfium.googlesource.com/pdfium/+/49dce65dc78bcd5a0c78a8bbdf2809cf20212220

TBR=thestig@chromium.org,weili@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:630654

Review-Url: https://codereview.chromium.org/2188523002
diff --git a/fpdfsdk/fsdk_mgr.cpp b/fpdfsdk/fsdk_mgr.cpp
index d1e5507..68143d2 100644
--- a/fpdfsdk/fsdk_mgr.cpp
+++ b/fpdfsdk/fsdk_mgr.cpp
@@ -353,12 +353,9 @@
   if (pPageView->IsLocked())
     return;
 
-  // Remove the page from the map to make sure we don't accidentally attempt
-  // to use the |pPageView| while we're cleaning it up.
-  m_pageMap.erase(it);
-
   pPageView->KillFocusAnnotIfNeeded();
   delete pPageView;
+  m_pageMap.erase(it);
 }
 
 UnderlyingPageType* CPDFSDK_Document::GetPage(int nIndex) {