This is an idea for a short term "fix" to some of our problems dealing with low memory.

The idea here is, until we get our tab discarder and/or OOM killer into good shape, we should prioritize OOM killing of Chrome background tabs over killing Android stuff.

Why?  Because killing a Chrome tab will actually free up a lot of memory as opposed to killing a dinky little Android background process.

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Specifically: the OOM killer works much worse than it used to before Android because we used to target a Chrome process to kill.  ...and almost always killing a Chrome process would free SIGNIFICANT memory.

Now, we kill a dinky Android process and it doesn't do much.  If we kill enough of them we might be able to get somewhere, but it takes a long time.

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Looking at a sample OOM kill on a kevin (memory pressure created by "balloon"), I see the following processes with a non -1000 OOM score:

<6>[  434.856591] [ pid ]   uid  tgid total_vm      rss nr_ptes nr_pmds swapents oom_score_adj name

<6>[  434.856877] [ 3476]  1000  3476    72387     9158     109       4     3197           300 chrome
<6>[  434.856886] [ 3624] 665368  3624   237778        0     106       4     3435           817 d.process.media
<6>[  434.856889] [ 3647] 665382  3647   236490        0      96       4     3041           650 hromium.arc.ime
<6>[  434.856896] [ 3739] 656360  3739   239527        0     100       4     3070           741 arc.applauncher
<6>[  434.856899] [ 3760] 665372  3760   275937     2522     182       4     6974           417 .gms.persistent
<6>[  434.856902] [ 3825] 665388  3825   240929        0     107       4     3329           665 rk.clouddpc.arc
<6>[  434.856905] [ 3836] 665372  3836   240556        0     108       4     3268           787 e.process.gapps
<6>[  434.856908] [ 3883] 665372  3883   312231      620     219       4     6654           772 gle.android.gms
<6>[  434.856911] [ 4063] 665362  4063   236299        0      99       4     3163           757 c.intent_helper
<6>[  434.856914] [ 4104] 665365  4104   235762        0     101       4     3036           954 viders.calendar
<6>[  434.856917] [ 4124] 665362  4124   236225        0      92       4     2968           726 arc.file_system
<6>[  434.856920] [ 4141] 665383  4141   236469        0      92       4     3004           711 hromium.arc.tts
<6>[  434.856923] [ 4178] 665392  4178   248077        0     121       4     8465           696 gle.android.tts
<6>[  434.856927] [ 4222] 665366  4222   237202        0     105       4     3352           802 d.process.acore
<6>[  434.856930] [ 4262] 665360  4262   236731        0      95       4     2983           680 crash_collector
<6>[  434.856933] [ 4274] 665372  4274   269743      384     167       4     4672           909 id.gms.unstable
<6>[  434.856936] [ 4331] 656361  4331   236299        0      99       4     3029           924 m.android.phone
<6>[  434.856938] [ 4398] 665372  4398   263267        0     143       4     4134           970 .android.gms.ui
<6>[  434.856942] [ 4465] 665372  4465   264094        0     147       4     4546           939 ndroid.gms:snet
<6>[  434.856945] [ 4967] 665372  4967   264845        0     149       4     4319           893 android.gms:car
<6>[  434.856948] [ 5018] 665367  5018   235098        0      93       4     2947           878 id.defcontainer
<6>[  434.856950] [ 5068] 665374  5068   235139        0      92       4     2937           848 android.musicfx
<6>[  434.856953] [ 5097] 656360  5097   235133        0      97       4     2943           863 ndroid.keychain
<6>[  434.856956] [ 5117] 665375  5117   262996    16862     164       4     7857           300 android.vending
<6>[  434.856959] [ 5132] 665389  5132   237581        0      97       4     3003           833 apps.cloudprint
<6>[  434.856962] [ 5206]  1000  5206   131227    23094     223       4    11880           533 chrome

When we pick an OOM victim, we end up picking:

<3>[  434.857036] Out of memory: Kill process 4398 (.android.gms.ui) score 971 or sacrifice child
<3>[  434.857142] Killed process 4398 (.android.gms.ui) total-vm:1053068kB, anon-rss:0kB, file-rss:0kB

Then we end up picking:

<3>[  437.829731] Out of memory: Kill process 4104 (viders.calendar) score 955 or sacrifice child
<3>[  437.842958] Killed process 4104 (viders.calendar) total-vm:943048kB, anon-rss:0kB, file-rss:0kB


These are just not terribly useful tasks to pick.  Although they may be "unimportant" and thus find to kill, killing them really doesn't provide any real benefit and doesn't help us free up memory.

It would be _much_ better if we instead chose to kill process 5206.  If we had picked that process to kill, we would have freed up 23094 pages (AKA 92MB of memory).

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This should be a very easy thing to do and could possibly even land in M-59.  teravest@ points to AdjustOomPrioritiesImpl() in Chrome as the place to change.

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Note that it's OK if the Chrome tab discarder doesn't change.  It could freely try to kill some Android processes.  ...but if the tab discarder fails, it seems like we should point the OOM killer at Chrome tabs before Android processes.