Add simpler reading methods to Blob interface.

[jarryd999]

Góðan dag TAG！

I'm requesting a TAG review of:

• Name: Simpler reading methods for Blob interface
• Explainer, Requirements Doc, or Example code: Add simpler reading methods to Blob interface. w3c/FileAPI#117
• Tests: https://crrev.com/c/1529716
• Primary contacts: @jarryd999, @mkruisselbrink

Further details (optional):

• I have read and filled out the Self-Review Questionnare on Security and Privacy. The assessment is not relevant for this feature.
• I have reviewed the TAG's API Design Principles

You should also know that this is an addition to existing major API that can easily be polyfilled.

We'd prefer the TAG provide feedback as (please select one):

• open issues in our Github repo for each point of feedback
• open a single issue in our Github repo for the entire review
• leave review feedback as a comment in this issue and @-notify [github usernames]

[mkruisselbrink]

Sorry, this is a duplicate of #359