[Cronet] Enable Brotli

- removes the compile time flag to disable brotli when building Cronet.
- adds an API to enable Brotli on CronetEngine.
- adds three basic java tests in BrotliTest.java

BUG=710633
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester

Review-Url: https://codereview.chromium.org/2805053005
Cr-Commit-Position: refs/heads/master@{#465216}
diff --git a/components/cronet/url_request_context_config_unittest.cc b/components/cronet/url_request_context_config_unittest.cc
index 7157a41..7b926e4e 100644
--- a/components/cronet/url_request_context_config_unittest.cc
+++ b/components/cronet/url_request_context_config_unittest.cc
@@ -29,6 +29,8 @@
       true,
       // Enable SDCH.
       false,
+      // Enable Brotli.
+      false,
       // Type of http cache.
       URLRequestContextConfig::HttpCacheType::DISK,
       // Max size of http cache in bytes.
@@ -130,6 +132,8 @@
       true,
       // Enable SDCH.
       false,
+      // Enable Brotli.
+      false,
       // Type of http cache.
       URLRequestContextConfig::HttpCacheType::DISK,
       // Max size of http cache in bytes.