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This CL adds a timer to WebApkIconHasher. Consequently, classes that
invoke WebApkIconHasher::DownloadAndComputeMurmur2Hash no longer need
to maintain a timer dedicated to WebApkIconHasher.

This CL also simplifies the error checking and handling logic.
WebApkIconHasher now checks for URL validity itself, and would return
an empty hash string for any errors encountered.

BUG=649771

Review-Url: https://codereview.chromium.org/2771793003
Cr-Commit-Position: refs/heads/master@{#460508}