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This reverts commit 733c2818.

Reason for reland: The initial change is actually an improvement, see
the linked bug.

Original change's description:
> Revert "android: Don't parse /proc/self/maps to prefetch the library."
>
> This reverts commit ef6f0fb3.
>
> Reason for revert: Regressed startup performance.
> See https://chromeperf.appspot.com/report?sid=9df0ec76b0fffc74931cc7bdcf44cd4d540f81bb34ccde3571c2211ec8360ee6&start_rev=531281&end_rev=531574
>
> Original change's description:
> > android: Don't parse /proc/self/maps to prefetch the library.
> >
> > This commit changes the behavior of the native library prefetcher to
> > rely on sentinel symbols rather than parsing /proc/self/maps.
> >
> > Behavior changes:
> > 1. This will make the code no longer prefetch .data (because we don't
> >    look for it in the mappings), neither the sections that are part of
> >    the same mapping as .text (for instance, .rodata when using
> >    ld.gold). This is intended.
> > 2. As a consequence, the UMA metric
> >    LibraryLoader.PercentageOfResidentCodeBeforePrefetch will move, as
> >    both sides of the ratio will change, and the population will change
> >    slightly as well (since code is currently not correctly ordered on
> >    ARM64 and x86_64)
> >
> > - Removes the reliance on a somewhat brittle parsing of a file
> > - Parsing /proc/self/maps costs ~50ms of CPU on a Nexus 5X (on a little
> >   core, as this is triggered from an AsyncTask)
> > - Allows to only fetch part of the native library (in a forthcoming CL)
> >
> > Rationale:
> > Change-Id: I0bb7b28af3c3bd4af5745e2ebcc1fbf283bcc0c1
> > Reviewed-on: https://chromium-review.googlesource.com/874470
> > Commit-Queue: Benoit L <lizeb@chromium.org>
> > Reviewed-by: Egor Pasko <pasko@chromium.org>
> > Reviewed-by: agrieve <agrieve@chromium.org>
> > Reviewed-by: Matthew Cary <mattcary@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#531515}
>
> TBR=pasko@chromium.org,agrieve@chromium.org,lizeb@chromium.org,mattcary@chromium.org
>
> Change-Id: I11d7cd7898d5f8d8fbebf391eba5fdaca154c08a
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/886321
> Reviewed-by: Benoit L <lizeb@chromium.org>
> Commit-Queue: Benoit L <lizeb@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#531858}

TBR=pasko@chromium.org,agrieve@chromium.org,lizeb@chromium.org,mattcary@chromium.org
Bug: 805883

Change-Id: Ida889088ec69c09f4dd95dd976174b5532811a53
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/886621
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Matthew Cary <mattcary@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531880}