Murmur2 hashes can take on values up to 2^64 - 1. 2^64 - 1 is larger than
Long#MAX_VALUE. This CL introduces
WebApkMetaDataUtils#getIconMurmur2HashFromMetaData() to extract the hash from
the meta data in the WebAPK's Android Manifest.

BUG=659348
TEST=WebApkMetaDataUtilsTest#testGetIconMurmur2HashFromMetaData,
     ManifestUpgradeDetectorFetcherTest.testIconMurmur2HashParsing

Review-Url: https://codereview.chromium.org/2452823002
Cr-Commit-Position: refs/heads/master@{#428140}