Remove ConcurrentHashMap from desugar list

When enable_jdk_library_desugaring=true, this change allows shrinking of
the generated "j$" dex from ~400 KB to 36 KB.

ConcurrentHashMap is available before before Java 8 but with a bug:
crbug.com/905903

Bug: 1056751
Change-Id: Ibaaeba2f714c0e8b17295cbe48098786ad173448
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446989Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Ian Wells <iwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814363}

third_party/r8/README.chromium

@@ -21,6 +21,7 @@ R8 is a proguard-like optimizer that also has the ability to dex.
 Local Modifications:
 * Refer to commit descriptions within "patches" directory.
 * Added "playground" directory for quick "how does this optimize" tests.
+* Removed references to ConcurrentHashMap in desugar_jdk_libs.json.
 
 Update Instructions:
 # Download R8:
@@ -57,3 +58,7 @@ java -cp third_party/r8/lib/r8.jar com.android.tools.r8.BackportedMethodList --m
 # Manually update:
 * README.chromium (version number via "java -cp lib/r8.jar com.android.tools.r8.R8 --version")
 * //DEPS (instance ID output by cipd create)
+
+Update instructions for desugar_jdk_libs.json:
+* Find desugar.json as described above and copy to desugar_jdk_libs.json.
+* Remove all mentions of ConcurrentHashMap (and remove any trailing commas if necessary).

third_party/r8/desugar_jdk_libs.json

@@ -118,8 +118,7 @@
         "java.util.Tripwire": "j$.util.Tripwire",
         "java.util.concurrent.DesugarUnsafe": "j$.util.concurrent.DesugarUnsafe",
         "java.util.concurrent.ThreadLocalRandom": "j$.util.concurrent.ThreadLocalRandom",
-        "java.util.concurrent.atomic.DesugarAtomic": "j$.util.concurrent.atomic.DesugarAtomic",
-        "java.util.concurrent.ConcurrentHashMap": "j$.util.concurrent.ConcurrentHashMap"
+        "java.util.concurrent.atomic.DesugarAtomic": "j$.util.concurrent.atomic.DesugarAtomic"
       },
       "retarget_lib_member": {
         "java.util.Arrays#stream": "java.util.DesugarArrays",
@@ -190,8 +189,7 @@
         "java.util.Spliterator": "j$.util.Spliterator",
         "java.util.StringJoiner": "j$.util.StringJoiner",
         "java.util.concurrent.ThreadLocalRandom": "j$.util.concurrent.ThreadLocalRandom",
-        "java.util.concurrent.atomic.DesugarAtomic": "j$.util.concurrent.atomic.DesugarAtomic",
-        "java.util.concurrent.ConcurrentHashMap": "j$.util.concurrent.ConcurrentHashMap"
+        "java.util.concurrent.atomic.DesugarAtomic": "j$.util.concurrent.atomic.DesugarAtomic"
       },
       "retarget_lib_member": {
         "java.util.Arrays#stream": "java.util.DesugarArrays",
@@ -239,9 +237,6 @@
     }
   ],
   "shrinker_config": [
-    "-keepclassmembers class j$.util.concurrent.ConcurrentHashMap$TreeBin { int lockState; }",
-    "-keepclassmembers class j$.util.concurrent.ConcurrentHashMap { int sizeCtl; int transferIndex; long baseCount; int cellsBusy; }",
-    "-keepclassmembers class j$.util.concurrent.ConcurrentHashMap$CounterCell { long value; }",
     "-keepclassmembers enum * { public static **[] values(); public static ** valueOf(java.lang.String); }",
     "-keeppackagenames j$",
     "-keepclassmembers class j$.util.IntSummaryStatistics { long count; long sum; int min; int max; }",