[Android] Add support for external sharding to the test runner.

testing/buildbot/chromium.swarm.json

Index: testing/buildbot/chromium.swarm.json
diff --git a/testing/buildbot/chromium.swarm.json b/testing/buildbot/chromium.swarm.json
index 70f448cd81e0b64b85440a6717a1ea94853619ac..1682bfd6292687eaf6fe7a8e0d48d8fc755c45a4 100644
--- a/testing/buildbot/chromium.swarm.json
+++ b/testing/buildbot/chromium.swarm.json
@@ -6,6 +6,7 @@
           "android_webview_test_apk"
         ],
         "override_isolate_target": "android_webview_test_apk",
+        "shards": 7,

[bpastene, 2017/03/14 21:02:13]

I'm pretty sure "shards" should go in the "swarming" dict. At least, that's how
your single larng tryjob did it

[jbudorick, 2017/03/14 22:15:16]

yep, fixed.

         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
@@ -35,6 +36,7 @@
           "chrome_public_test_apk"
         ],
         "override_isolate_target": "chrome_public_test_apk",
+        "shards": 14,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
@@ -48,6 +50,7 @@
       },
       {
         "override_isolate_target": "content_browsertests",
+        "shards": 10,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
@@ -61,6 +64,7 @@
       },
       {
         "override_isolate_target": "content_unittests",
+        "shards": 3,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
@@ -74,6 +78,7 @@
       },
       {
         "override_isolate_target": "net_unittests",
+        "shards": 4,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
@@ -87,6 +92,7 @@
       },
       {
         "override_isolate_target": "unit_tests",
+        "shards": 5,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
@@ -107,6 +113,7 @@
           "android_webview_test_apk"
         ],
         "override_isolate_target": "android_webview_test_apk",
+        "shards": 7,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
@@ -136,6 +143,7 @@
           "chrome_public_test_apk"
         ],
         "override_isolate_target": "chrome_public_test_apk",
+        "shards": 15,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
@@ -149,6 +157,7 @@
       },
       {
         "override_isolate_target": "content_browsertests",
+        "shards": 10,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
@@ -162,6 +171,7 @@
       },
       {
         "override_isolate_target": "content_unittests",
+        "shards": 4,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
@@ -175,6 +185,7 @@
       },
       {
         "override_isolate_target": "net_unittests",
+        "shards": 4,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
@@ -188,6 +199,7 @@
       },
       {
         "override_isolate_target": "unit_tests",
+        "shards": 8,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [