Add value() and name() methods to enum types. (#1775)

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:
```
synthtool > Executing /tmpfs/src/git/autosynth/working_repo/ErrorReporting/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:854131ec1af7b3a313253474c24748dc0acd217a58a0b74dbfb559f340a15d78
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/devtools/clouderrorreporting/artman_errorreporting.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-error-reporting-v1beta1.
synthtool > Replaced 'Copyright \\d{4}' in src/V1beta1/Gapic/ErrorGroupServiceGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1beta1/Gapic/ErrorStatsServiceGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1beta1/Gapic/ReportErrorsServiceGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1beta1/ErrorGroupServiceClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1beta1/ErrorStatsServiceClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1beta1/ReportErrorsServiceClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1beta1/ReportErrorsServiceClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1beta1/ErrorStatsServiceClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1beta1/ErrorGroupServiceClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/System/V1beta1/ReportErrorsServiceSmokeTest.php.
synthtool > Cleaned up 1 temporary directories.
synthtool > Wrote metadata to synth.metadata.

```

ErrorReporting/synth.metadata

@@ -1,19 +1,19 @@
 {
-  "updateTime": "2019-03-28T10:00:57.378625Z",
+  "updateTime": "2019-03-29T10:00:46.339936Z",
   "sources": [
     {
       "generator": {
         "name": "artman",
-        "version": "0.16.20",
-        "dockerImage": "googleapis/artman@sha256:e3c054a2fb85a12481c722af616c7fb6f1d02d862248385eecbec3e4240ebd1e"
+        "version": "0.16.21",
+        "dockerImage": "googleapis/artman@sha256:854131ec1af7b3a313253474c24748dc0acd217a58a0b74dbfb559f340a15d78"
       }
     },
     {
       "git": {
         "name": "googleapis",
         "remote": "https://github.com/googleapis/googleapis.git",
-        "sha": "6a84b3267b0a95e922608b9891219075047eee29",
-        "internalRef": "240640999"
+        "sha": "37386f1ebcaed51bcb7d15d00124d22c38606e45",
+        "internalRef": "240841842"
       }
     }
   ],